51 Pegasi was the first Sun-like star found to have a planet in orbit around it. Located in the constellation of Pegasus, it lies just over 50 light-years from our Solar System.
The star has an apparent magnitude of 5.49 and is visible with the naked eye under suitable conditions. With an effective temperature of 5571 K, the star has a yellow hue. It is thought to be slightly older than the Sun with an age in the range of six to eight billion years old, and it is also 24% larger and 11% more massive. This gives it a higher proportion of other elements beside hydrogen and helium.
The discovery of a planet in orbit around 51 Pegasi was made in 1995, and was given the informal name of Bellerophon. It is known to orbit very close to the host star, and temperature estimates of the surface show that it is around 1200°C. The minimum mass is thought to be around half that of Jupiter.
Achernar (Alpha Eridani)
Achernar (sometimes spelt Achenar) is the hottest and bluest star of all the brightest stars in the sky. It also lies at the southern tip of the Eridanus constellation.
The star is between six and eight solar masses, and located 139 light-years from the Earth.
One of the remarkable properties of the star is that it is the least spherical currently known in the Milky Way, with the equatorial diameter being 56% greater than the polar diameter. This is due to the extremely rapid spin of Achernar at 250 km/s. This distortion of the star also causes major changes in the temperature of the surface; with the pole above 20,000 K and the equator at about 10,000 K. The average temperature of the surface is around 15,000 K.
Acrux (Alpha Crucis)
Acrux is the brightest star in the constellation of Crux, with a magnitude of 0.77. It forms the South Cross along with stars Mimosa, Pálida, Becrux, and Gacrux.
The star of Acrux is actually a star system at a distance of 321 light-years from Earth. Only two components of the star system are distinguishable, a 1 and a 2 which have surface temperatures of 28,000 K and 26,000 K respectively.
The primary and secondary stars have significantly greater masses than the Sun, at around 14 and 10 times. It is suggested that both stars will go supernova in the future. The third component of the star system shares Acrux’s motion through space, which suggests that it is gravitationally bound to the primary and secondary stars.
Australia, New Zealand, and Papua New Guinea all share a common fact in that Acrux is represented on the flag of each country. Brazil also uses Acrux as one of 26 stars to represent each state, with Acrux representing São Paulo.
Adhara (Epsilon Canis Majoris)
Adhara is the second brightest star in the constellation of Canis Major regardless of its Epsilon designation. In some cases, it may be named Adara, which comes from the Arabic word for ”virgins”.
Adhara is a binary star, which lies 430 light-years from Earth. The main star has an apparent magnitude of 1.5, with a colour of blue or blue-white. This indicates its high temperature of 22,200 K, and it emits a total radiation of 38,700 times that of the Sun.
The companion star has a magnitude of 7.5, which can only be resolved in large telescopes as the main star is over 250 times brighter.
At one time, Adhara was much closer to the Sun. Around 4.7 million years ago, it was 34 light years from the Sun and would have been the brightest star in the sky with a magnitude of -3.99; a brightness which no other star has achieved since (except the Sun), and which no other star will achieve for another five million years.
Aldeberan (Alpha Tauri)
Located in the Taurus constellation, Aldeberan is a red giant star located about 68 light-years from Earth. It has an apparent magnitude of 0.87 meaning it is one of the brightest stars in the sky.
The star is sometimes referred to as the follower, as it appears to follow the Pleiades star cluster as it moves across the sky.
At 3,900 K in temperature, Aldeberan has exhausted its supply of Hydrogen fuel however is not yet hot enough to fuse Helium. The core temperature has significantly increased however due to the pressure of gravity. This has caused the star to expand to 44 times the diameter of the Sun.
Aldeberan is relatively easy to find in the night sky as it is close to Orion’s Belt. It also has an appearance of being a part of the Hyades open star cluster, however its position here is just by chance of being in the line of sight between Earth and the star cluster.
In 1993, measurements taken of Aldeberan appeared to indicate it had a companion in orbit. Some suggestions were made that it was a planet, however the data indicated that the planet would have a mass eleven times that of Jupiter, and with an orbit of 643 days. More recent surveys have concluded that the measurements were simply a feature of the star as opposed to a body in orbit.
Algol (Beta Persei)
Algol, nicknamed the Demon Star is located within the Perseus constellation. It is actually a star system consisting of three stars in which Algol B orbits Algol A.
Most of the time, Algol has a magnitude of 2.1, however this dips every two days to 3.4. The two stars are a distance of 0.0062 astronomical units, with the third star being 2.69 AU on average. The orbital period is just over 2.8 days for Algol B, and 680 days for Algol C.
Currently, the Algol system is located 92.8 light-years from the Sun, however 7.3 million years ago it passed within 9.8 light-years from our Solar System. It is believed that with the star systems combined mass of 5.8 solar masses, the close proximity on which it made its closest pass would have been enough to disturb the Oort Cloud.
Alpha Centauri, sometimes known as Rigel Kent is a star system that is the closest to the Sun, at 4.37 light-years away. The star system is composed of two stars forming a binary system, and a third companion star which create a visible magnitude of -0.27, making it the third most luminous ”star” in the sky.
The individual stars of Alpha Centauri are Alpha Centauri A which is 110% the mass of the Sun, around 6152% the luminosity. Alpha Centauri B is just over 90% the mass of the Sun, and 44.5% as luminous.
A third star, Proxima Centauri, is gravitationally associated to Alpha Centauri AB, and is the nearest individual star to our Sun. It orbits AB at around 15,000 AU, and our best estimate is that is completes an orbit of its hosts between 100,000 and 500,000 years.
At least one planet is known to orbit Alpha Centauri. It is 113% the size of Earth, however completes a single orbit just over every three Earth days. This means that it is extremely close to its host star, orbiting at around 4% the distance that Earth does from the Sun and the surface temperature is around 1,500K.
Alphard (Alpha Hydrae)
Located in the constellation of Hydra, Alphard is the brightest star. The name is from the Arabic language, and means ”the solitary one” due to the lack of bright stars near it. It is also known as the ”backbone of the serpent” to Arabs.
Alphard has three times the mass of the Sun and an estimated age of 420 million years. During this time it has evolved away from being a main sequence star into a red giant. This caused it to swell to 50 times the radius of the Sun.
Spectrum analysis of Alphard shows that the star contains an excess of barium. This is unusual in that barium is often found in binary star systems, however it is thought that the cause of this was the transition away from a white dwarf.
Alphard also appears on the flag of Brazil where it represents the state of Mato Grosso do Sul.
Alpheratz (Alpha Andromedae)
Alpheratz is the traditional name for Alpha Andromedae or Sirrah. It shares the honour of being the brightest star in the Andromeda constellation with Mirach.
The star is located 97 light-years from Earth, and although it appears as a single star, it is actually a binary star system in which the stars orbit each other every 96.7 days. Composed of high levels of mercury and manganese, it is unusual for a star to contain such levels of those elements. This feature also makes it the brightest known mercury-manganese star.
The larger, brighter star has a mass 3.6 times that of the Sun and a surface temperature of 13,600 K. It is also around 200 times more luminous than the Sun. The smaller companion is 1.8 times more massive than the Sun, and has a temperature of 8,500 K, with a luminosity 10 times the Sun’s.
The star is very slight variable, with a difference of less than 0.01. This is caused by clouds of mercury concentrate at the equator which drift slowly over the surface of the star.
Altair (Alpha Aquilae)
Altair is contained within the constellation of Aquila, and is considered the twelfth brightest star in the night sky with an apparent visual magnitude of 0.77.
It is located 16.7 light-years from Earth, meaning that it is one of the closest stars visible with the naked eye. Together with Beta Aquilae and Gamma Aquilae, the star forms a line of stars known as the Family of Aquila.
Altair has a mass 1.8 times greater than the Sun, but its luminosity is eleven times greater. It also has an extremely rapid rate of rotation, with one rotation taking around nine hours. This differs from the Sun significantly which takes 25 days to rotate. The speed of rotation on Altair means that the diameter of its equator is over twenty percent greater than the polar diameter.
Antares (Alpha Scorpii)
Antares, also known as Alpha Scorpii is a red supergiant and is the sixteenth brightest star in the night sky with a visible magnitude of 1.05. The star is also a part of the group Scorpius-Centaurus which contains many stars with an average age of 11 million years and at a distance of 470 light-years.
The star has a radius about 880 times that of the Sun, and if placed at the centre of our Solar System, the orbit would extend out to the Asteroid Belt.
Another star orbits Antares as a companion, and is known as Antares B. Estimates suggest that it orbits its parent star every eight hundred and seventy eight years. At the end of May, Antares can be viewed all night as it is in opposition to the Sun. The star sets in the morning and rises in the evening. Equally, the star is not visible for parts of November as it is near conjunction with the Sun.
Arcturus (Alpha Böotis)
The fourth brightest star in the night-sky is Arcturus with an apparent magnitude of -0.04. The star is relatively close to Earth at a distance of 36.7 light-years.
The star is classed as an orange giant. It has likely burnt through its reserve of hydrogen fuel and has begun fusing the remaining helium into oxygen and carbon in its core. The star will then expand before shedding its outer shell to end its life as a white dwarf.
Arcturus is located close to the equator of Earth, so is visible from both hemispheres. At one hundred and ten times most luminous than the Sun, however this is an underestimate of the brightest as a large proportion of the light given off is in the infra-red spectrum. The lower output of visible light is due to the surface temperature of the star being lower than the Sun.
It is possible that Arcturus is a binary star with a companion many times dimmer than the primary. If the companion does exist, it is orbiting close enough that it is at the very limit of our ability to make it against the brightness of the host. In 1993, further measurements were taken which neither confirmed or denied the presence of a companion however it is believed that the orbit oscillations may be a property of the star rather than an orbiting companion.
Barnard’s Star is a low-mass red dwarf located about six light-years from our Solar System. It is the fourth closest star to our Sun, however can be considered second closest after counting the three belonging to the Alpha Centauri system.
Even though it is relatively close, it has a low visible magnitude of 9.5 and is therefore unable to be viewed by the naked eye. However, the star is extremely bright in infrared compared to visible light.
The star is considered to be between seven and twelve billion years old and is possibly one of the oldest stars in the Milky Way. It has however lost a significant amount of rotational energy, and only rotates once every 130 days (the Sun rotates every 25 days).
With around 14% the mass of the Sun and a radius between 15% and 20%, Barnard’s star is relatively small. In fact, it is believed to be up to only twice the size of Jupiter with regards to its radius.
Bellatrix (Gamma Orionis)
Bellatrix is designated as the third brightest star in the constellation of Orion. It is also the twenty seventh brightest star in the sky.
The star is 8.4 times the Sun’s mass, and has an approximate age of twenty million years. In this time, it has burnt through its reserves of hydrogen and evolved into giant with a surface temperature of 22,000 K.
This extreme temperature gives the star a blue-white hue. The star was originally included along with a bright set of stars that were utilised as a standard for luminosity. The apparent magnitude at the time was measured at 1.64. However, further tests in 1988 revealed that the magnitude had changed to 1.59 which showed that Bellatrix itself was a variable star.
Betelguese (Alpha Orionis)
Betelguese is the eight-brightest star in the night-sky, and despite is designation of alpha, the second brightest in the constellation of Orion.
The star is classified as a red supergiant, and if placed at the centre of our Solar System, its surface would extend past the Asteroid Belt. However, the star also has a large atmosphere surrounding it, with its photosphere reaching to the orbit of Saturn. Estimations of its mass vary wildly with values between five and thirty times that of the Sun.
It is expected that Betelguese will go Supernova in the next million years as the star is considered old for its size. Once fusion has stopped, and its core has turned to Iron, the star will explode leaving a neutron star remnant around 20 kilometers in diameter.
Betelguese is also travelling at supersonic speeds through space, after having been ejected from its birth place nearer to Orion’s Belt. This speed creates a bow shock as the stellar wind it ejects leaves at 17 kilometres per second. This bow shock has only recently been observed due to the brightness of the star and the size of its atmosphere.
Canopus (Alpha Carinae)
With a magnitude of -0.72, Canopus is the brightest star in the constellation of Carina and Argo Navis. It is also the second brightest star in the night sky after Sirius. The star is located in the southern sky and is visible in the northern hemisphere as far north as Seville, Spain.
The star is blue-white in colour, and has a surface temperature estimated to be 7,350 K. For large parts of the last four million years, Canopus was actually the brightest star. It is currently the second brightest behind Sirius; this will remain the case for the next two hundred thousand years until Canopus regains its status as the brightest again.
If placed at the centre of our solar system, the surface would extend almost to the orbit of Mercury. It is located around 310 light-years from Earth.
The corona of Canopus is a strong source of X-rays, and the temperature has been measured at around fifteen million kelvin.
Capella (Alpha Aurigae)
Capella is the brightest star in the constellation of Auriga, and the eleventh brightest star in the night sky of the northern hemisphere. Although it appears to be a single star, it is actually a system of four stars; two binary pairs.
The first pair of stars are around ten times the radius of the Sun, and are large type-G giant stars. These stars are thought to be cooling, before which they will expand to become red giants. The second pair of stars are around 10,000 AU from the first pair and consist of two faint, small red dwarfs.
At around 42 light-years from Earth, the Capella star system is relatively close to our Solar System. Capella appears to be a yellow colour, and between 210,000 and 160,000 years ago, it would have been the brightest star in the night sky. It is also the closest star to the north celestial pole of all first magnitude stars. This means that the star never sets when viewed from the United Kingdom, north France, Canada, and Scandinavia.
Castor (Alpha Geminorum)
Castor is the second brightest star in the Gemini constellation, however despite its alpha designation it is actually fainter than Pollux.
Located 49.8 light-years away from Earth, the star was discovered to be a part of a binary system in 1678. The period of rotation for the stars is 467 years. Later discovered were additional stars in orbit around each primary, and then two more forming a sextuple star system.
The apparent magnitude of the star system are 2.0 and 2.9, with a combined magnitude of 1.58.
Cor Caroli (Alpha Canum Venaticorum)
Cor Caroli is the brightest star in the northern constellation of Canes Venatici.
It is a binary system consisting of two stars which are distinctly separated. It has an apparent magnitude of 2.81 with the stars being 19.6 arcseconds apart. This means that the two stars can be seen through smaller telescopes. The system also lies 110 light-years from Earth.
a2 is the brighter of the two stars, however it is a peculiar star as it has high amounts of silicon, mercury and europium. The cause of this is believed to be that some elements have sank due to gravity while others elevated. The star also has a strong magnetic field.
a1 is many times dimmer than its companion and has an apparent magnitude of 5.60. Its rotational velocity is also considerably slower at 8 km/s.
Deneb (Alpha Cygni)
Deneb is one of the most luminous stars in the night-sky with an apparent magnitude of 1.25. It has been difficult however to calculate the distance to the star from Earth, therefore it is thought that the star is between 54,000 and 196,000 times more luminous than the Sun.
The star is located in the constellation of Cygnus, and is given the Alpha designation as it is the brightest. It is also a part of the summer triangle, which also consists of stars Vega and Altair. The triangle can often be seen in the northern sky as there are few other bright stars in the vicinity.
Based on best known calculations, the diameter of Deneb is between one and two hundred times greater than the Sun, and if placed at the centre of the solar system, its surface would extend out to the orbit of Earth.
As a blue-white supergiant star, with a temperature of 8,500 K, Deneb will be short-lived and is expected to go supernova within several million years. Hydrogen fusion in the core has already stopped, and the star is thought to be slowly expanding into a red supergiant.
Denebola (Beta Leonis)
Denebola is the second brightest star in the constellation of Leo, and is located 36 light-years from Earth.
It is a A-type main sequence star, and is 75% more massive than the Sun. The luminosity is also twelve times greater than that of the Sun. Its visual magnitude is 2.14.
The star is relatively young with an age somewhere in the region of 400 million years old. It has a high rate of rotation, at around 128 km/s which creates an equatorial bulge. The rate of rotation is also similar to Achernar.
With a temperature of around 8,500 K, Denebola has a white hue which is common among stars of this type. Imaging has revealed that Denebola appears to have a disk of debris in orbit around it which is similar to that which our Solar System formed out of. It is believed that Denebola will be a good candidate for extrasolar planets.
Located in the constellation of the Carina, at a distance of around 7,500 light-years from Earth, Eta Carinae consists of two stars. It is thought that initially the star system had around 150 solar masses, however it mass has dropped to around 120 solar masses.
The companion star is not able to be seen optically due to the thick red nebula surrounding Eta Carinae. It is suggested that the companion has a mass of 30 solar masses however, and is classed as a supergiant.
The discovery that Eta Carinae was a binary system was only made in 2005, as prior suggestions were that it could be a single massive star. The primary star has a mass of around 90 solar masses and produces around one million times more light than the Sun. The vast properties of the primary Era Carinae star means that it is thought the object approaches the Eddington Limit, where the outward bound radiation would be strong enough to counteract the gravity holding the object together.
Due to the large size of Eta Carinae, the star is highly likely to explode in a supernova or hypernova. Potentially, the explosion could affect the outer atmosphere of Earth as cosmic rays hit the planet. Satellites and spacecraft many face significant danger however due to this lack of protection. Some suggestions also predict that a supernova or hypernova explosion would be visible during the day on Earth.
Gacrux (Gamma Crucis)
Gacrux is located 88.6 light-years from our solar system, and is also the nearest red giant star from our Sun. The star is located at the top point in the Southern Cross.
With a magnitude of 1.63, Gacrux is one of the brightest stars in the sky. The star evolved into a red giant from a main sequence star, however it is only around 30% more massive than the Sun. It is thought that at this point, the star has expanded to 84 times the radius of the Sun, and emits 1,500 times the luminosity.
The atmosphere of Gacrux is rich in barium, and the surface has an effective temperature of just over 3,600 K. This gives the star a red-orange appearance, which is typical of stars of this type.
As with the other stars in the Southern Cross, the star is represented on the flags of Australia, New Zealand, and Papua New Guinea. The Brazilian flag also represents the star, as the star represents the state of Bahia.
Fomalhaut (Alpha Piscis Austrini)
Fomalhaut is located about 25 light-years from our Solar System. It is a relatively young star at around four hundred and forty million years old. It is believed to be halfway through its life however as it the projected lifespan is about one billion years old.
The surface temperature is 8,600 K, and the mass is 1.9 times that of the Sun. Its diameter is 16.6 times greater, and the diameter is 1.8 times as large.
Fomalhaut is surrounded by several disks of debris. The first is located at 0.1 AU from the star and contains small grains of carbon. The second disk is made-up of larger particles between 0.4 AU and 1 AU. The third and outermost disk is at a distance of 133 AU from the star, and emits a large amount of infrared radiation.
In 2008, it was announced that Fomalhaut had an extrasolar planet orbiting it. This was the first planet to be discovered by visible light, and was found using the Hubble Space Telescope. The planet is named Fomalhaut B. There have been some doubts however about the construct of the planet, with some suggesting that it may be a dust cloud in orbit around the star.
Hadar (Beta Centauri)
Hadar, often called Agena or Beta Centauri is a star system in the constellation of Centaurus. The name Hadar comes from the Arabic of ground or soil, whereas Agena comes from Latin and means ”the knee”.
It has an apparent visual magnitude of 0.60 and is the second brightest star in the constellation. It is also the tenth brightest in the sky.
Based on parallax measurements, the system is located around 350 light-years from Earth. It is believed that the primary star has exhausted its hydrogen reserves and has evolved away from a main sequence star.
The belief that the a star system existed dates back to 1967, and this was confirmed in 1999. The primary consists of two stars of near-identical mass which orbit each other at a distance of around four astronomical units.
Mimosa (Beta Crucis)
Mimosa is often known by two other names; Beta Crucis and Becrux. It is the second brightest star in the constellation of Crux, with Acrux being the brightest. It also forms the left point of the South Cross, and appears on the flags of several nations (Australia, Brazil, New Zealand, Papua New Guinea).
Mimosa is thought to be the hottest first-magnitude star, and has an apparent magnitude of 1.25. It is located around 280 light-years from Earth, with the distance being discovered using the parallax measuring technique.
The primary star is a massive star of around 16 times greater mass than the Sun. It also completes a rotation every 3.6 days. The effective temperature is thought to be 27,000 K, giving the star a blue-white glow. It also emits a solar wind with a velocity of 2,000 km/s. The second companion star is thought to be a main sequence star, and a third companion star which was found in 2007 may be a pre-main sequence star.
Mirach (Beta Andromedae)
Mirach is located in the Andromeda constellation, and is the brightest star within the group along with Alpheratz. The name Mirach is the traditional name, however alternative spellings are Merach, Mirak, and Mirac.
Mirach has an apparent magnitude of 2.07, however this does vary between 2.01 and 2.10. The variety in magnitude is caused by gas and dust littering the line of sight to the star. The star has also expanded into a red giant to a size around 100 times larger than the Sun.
The star radiates 1,995 times more luminosity than the Sun, and has an effective temperature of 3,842K. Using parallax measurements, the star is also located 197 light-years from Earth.
Mirphak (Alpha Persei)
Mirphak is known by several names and variations including Mirfak, Alpha Persei, and Algenib. The visual magnitude is around 1.8 and the thirty seventh brightest in the sky.
The star is located in the constellation of Perseus and has an alpha designation. It is also located around 510 light-years from our solar system.
At around 7.3 times the mass of the Sun, Mirphak is over 5,000 times more luminous and has a temperature of 6,350 K creating a yellow-white glow.
Mirzam (Beta Canis Majoris)
Mirzam, known by its official name of Beta Canis Majoris is a bright star in the constellation of Canis Major. It also has the alternative names of Murzim and Al-Murzim.
It is located around 500 light-years from our solar system, and can be seen on the head of the constellation. The age of the star is thought to be around 12 million years old. This is old enough for the star to have begun to evolve into a giant star.
Mirzam has an apparent magnitude between 1.95 and 2.00 over a six-hour period, however the change is too small be perceived by the naked eye. The effective temperature is around 23,150 K , which is around four and a half times that of the Sun.
The Brazilian state of Amapà is represented by Mirzam, and the star appears as one of twenty six on the flag.
Mu Cephei, also known as Herschel’s Garnet Star is a red supergiant star. It is located in the Cepheus constellation and one of the most luminous and largest stars in the Milky Way. It is also 100,000 times more luminous than the Sun, however it has a visual magnitude of 4.08.
The star is garnet red in colour, due to its lower range temperature of around 3700 K. However it is thought that the star may be surrounded by a shell of gas, with the gas temperature being measured at around 2,050 K. The shell is thought to be around 2,500 years old.
At approximately 1,000 times larger than the Sun, Mu Cephei if placed at the centre of the Solar System would extend out to the orbit of Jupiter.
The distance Mu Cephei is located from Earth still has not been accurately determined. Comparisons with Betelguese suggest that it could be 390 parsecs away, however this contains a large margin of error. It is suggested that either Mu Cephei is either much larger than Betelguese or much closer (therefore smaller and less luminous) than expected.
It is known that Mu Cephei is nearing death as it has begun fusing Helium into Carbon, indicating that the star may go supernova relatively soon (astronomically speaking), although this could be still millions of years away from occurring.
NML Cyngi is a red hypergiant star, and currently the largest known star with a radius 1,650 times that of the Sun. If the star was placed at the centre of our Solar System, its atmosphere would extend to the gap between the orbits of Jupiter and Saturn, at around 9.6 AU. The star also has a dusty environment surrounding it.
The star is located around 5,300 light years from Earth as was discovered in 1965. Even though the star is so large, the apparent magnitude is well outside the visible range of the naked eye at 16.6 due to the vast distance it lies away. It is considered however to be one of the most luminous stars of its type.
The composition of the atmosphere is oxygen rich, however other molecules have been found including water, carbon monoxide, sulphur dioxide, and hydrogen sulphide.
Pálida (Delta Crucis)
Pálida, meaning “pale one” in Portuguese is a southern star in the constellation of Crux. It is represented on the flags of Australia, New Zealand, and Papua New Guinea. It also represents the state of Minas Gerais in Brazil.
The star has an apparent magnitude of 2.79, and is the faintest of the four bright stars making up the points of the Southern Cross. It is located at a distance of 345 light-years (106 parsecs) from Earth.
Pálida is a massive star which is currently undergoing the process of evolving into a giant. Currently, the star is classed as a subgiant. The rotation rate is also extremely fast with a rotational velocity of around 210 km/s.
The star is 10,000 times more luminous than the Sun and measurements indicate that the surface temperature is 22,570 K. This high temperature gives it a blue-white hue. The star is variable however, and its brightness changes subtly every 1.3 hours.
The Pistol Star is a blue hypergiant, and is one of the most luminous stars in the Milky Way galaxy. The star is relatively young at around four million years old, and it is thought that it will go supernova sometime between one and three million years into the future. The star is actually part of a group of young stars which have formed near the centre of the galaxy.
Discovered in the early 1990s, the star is believed to have ejected ten solar masses in gaseous outbursts around 4,000 and 6,000 years ago. The stellar wind is around ten billion times stronger than that of the Sun.
The luminosity of the star is so great that it was once thought to be the most luminous star, however it was later discovered that the Eta Carinae system was three times brighter. Even so, the star radiates more energy in twenty seconds as the Sun does in one year.
Polaris (Alpha Ursae Minoris)
Polaris is known as the Pole Star due to its position at the north celestial pole. For an observer in the northern hemisphere, it appears that all other stars rotate around Polaris. It is also known as the North Star or Guiding Star. The star is also the brightest star in the constellation Ursa Minor.
The distance to Polaris is calculated at about 434 light-years, however some suggestions put it about 30% closer.
Polaris is actually made up of a star system consisting of five stars. The main star; Polaris A is around 4.5 solar masses. The two smaller companions have solar masses of 1.39 and 1.26 making them only slightly larger than the Sun. The final two stars, Polaris C and Polaris D orbit their host at a distance and are small; therefore little is known about their composition.
Pollux (Beta Geminorum)
Pollux is a star of beta designation in the constellation of Gemini. The star has a visual magnitude of 1.1, which is brighter than alpha designated star, Castor.
The star is located at just under thirty four lights years from Earth, and in 2006 was confirmed to have an extrasolar planet in orbit. The mass of the planet is 2.3 times that of Jupiter, and has an orbit lasting 590 days.
Pollux has exhausted the hydrogen in its core, and evolved into a giant star. The temperature of the outer atmosphere is over 4,600 K and produced an orange colour.
An abundance of elements within Pollux has been detected, however estimates range from 85% to 155% that of the Sun.
Procyon (Canis Minoris)
Procyon is one of the stars making up the Winter Triangle. With a mass not too different to the Sun at 1.4 times, and a radius twice the size, Procyon is actually almost seven times more luminous.
Due to the brightness of the star compared to others in the same class, it is thought that the reserves of hydrogen have nearly been fused into helium. This is causing Procyon to expand, and is estimated to swell to between 80 and 150 times its current size.
Procyon also has a companion star which is a white dwarf. The visual discovery of the companion star as first made in 1896, and was found using a 36-inch refracting telescope at the Lick Observatory. The star is less massive than the Sun, at 0.6 solar masses.
Regulus (Alpha Leonis)
Regulus is locate in the constellation of Leo, where it is the brightest star. It is also one of the brightest in the night sky – and can also be seen during the day in the right conditions using a telescope. Its position in seventy-nine light years away from the Sun.
Consisting of four stars organised into pairs, Regulus A is the dominant star and is a blue-white main-sequence star, while B and C are dim main-sequence stars.
Regulus A is also orbited by a small star at 0.3 solar masses, and is thought to be a white dwarf though this has not been directly observed. Regulus A is around three times the Sun’s mass, and has a rotation period of just short of sixteen hours – causing it to have an oblate shape. This causes gravity darkening and as such means that the poles are hotter and brighter than the equatorial region. The speed of rotation is also the reason the star emits polarised light.
Rigel (Beta Orionis)
The seventh brightest star in the night sky is Rigel, which is located in the constellation of Orion. The luminosity is thought to be between 120,000 and 279,000 that of the Sun.
Actually a multiple star system of three to five stars, the primary, Rigel A, is a blue-white supergiant. Its core of hydrogen has now been exhausted, causing it to swell to up to one hundred and fifteen times the radius of the Sun and it is also pulsates. Rigel B is significantly fainter than A, but visible with a telescope. It is a binary system itself with two main sequence blue-white stars. Rigel C is almost identical in appearance to Rigel B.
It has been measured that Rigel is 863 light-years away from our own solar system. This measurement was made in 2007, and revised downwards from an earlier distance of 1,200 and 1,600 light-years.
Saiph (Kappa Orionis)
As the sixth brightest star in the constellation of Orion, Saiph is actually made up of four bright stars. The name Saiph comes from Arabic and translates literally as ‘sword of the giant’. This name however was originally applied to Era Orionis. It has been aged at 6.2 million years old.
Saiph is positioned at around 650 light-years from the Sun which is roughly the same as that of Betelguese. It also has a strong stellar wind and loses a significant amount of mass each year – around one Sun every 1.1 million years.
In comparison, it has a mass twenty-eight times that of the Sun. With regard to luminosity, Saiph is less luminous than Rigel and has an apparent visual magnitude of 2.09 though this does vary slightly.
Sirius holds the place as the brightest star in the Earth’s night sky, and is almost twice as bright as second brightest star Canopus, with an apparent visual magnitude of -1.46. The reason for the brightness is due to the relatively close proximity to Earth at 8.6 light years. It is also moving closer to the Solar System so will increase in brightness over the next 60,000 years, before it moves away again and will become fainter. Sirius is expected to be the brightest star in the night sky for the next 200,000 years in any case. Viewing through the day can also be made in the right conditions.
Perceived as a single star, Sirius is actually a binary system consisting of a white main-sequence star named Sirius A, and a fainter white dwarf named Sirius B. Sirius B in actuality was once the more massive of the two stars, however it has evolved off the main sequence to its current state. It is also one of the most massive white dwarf stars currently known. The distance between the two stars is estimated between 8.2 and 31.5 astronomical units (AU), with 20 AU being thought reasonable (about the same distance between the Sun and Uranus).
For some time it was believed that the Sirius system may consist of a third star, however this was ruled out by the Hubble Space Telescope after measurements found no irregularities in the orbits of Sirius A or B.
The name ‘Dog Star’ is often given to Sirius due to its prominence in the constellation Canis Major.
Spica (Alpha Virginis)
Spica is the brightest star in the Virgo constellation, and is actually is a binary star system which orbit each other every four years. The pair are so close that they can not be resolved separately through a telescope.
The primary star is midway between a subgiant and giant star – meaning it is no longer on the main sequence. It is around ten times more massive and seven times the Sun’s radius. Once it reaches its end of life, it is expected to result in a Type II supernova explosion and is one of the nearest stars to the Sun to end this way. The star also has a rapid rotational rate and has a property which causes it to regularly pulsate and contract causing variation in the brightness.
The secondary star is a main sequence star and subject to the Struve-Sahade effect which causes a change in the strength of the spectral lines as the star orbits. This results in lines becoming weaker as the star moves away from the observer and is believed to be caused by a strong stellar wind. The star is also smaller than the primary, with around seven times the mass of the Sun and over three times the Sun’s radius.
Vega (Alpha Lyrae)
Vega has the distinction of being the brightest star in the Lyra constellation, and the fifth brightest in the night sky. Together with Arcturus and Sirius it is one of the most luminous stars in the Sun’s neighbourhood.
Being around 2.1 times more massive than the Sun, it is expected to have a short lifetime and it is approaching the midpoint of its life – however it is also only a tenth the age of the Sun. It also has an unusually low number of elements with a higher atomic number than helium.
The star rapidly rotates causing the equator to bulge due to centrifugal force, which results in a large difference in the temperature across the photosphere with the highest temperatures being found at the poles.