Royal Astronomical Society of New Zealand
α1 + α2 Cen
α1 + α2 Cru
α Gem, double
The apparent visual magnitude of the Sun is -26.8, making it about 10 billion (10 000 000 000) times as bright as the next brightest star, Sirius. However its brighteness is due to its proximity to the Earth. Light takes about 500 seconds to travel from the Sun to our planet. The mean distance ot the Earth from the centre of the Sun is about 149 000 000 km, a distance known as an astronomical unit (AU). While astronomical units are useful for giving distances in the Solar System, they are of little use in expressing distance to stars as there are over 63 000 AU in 1 light year. That is 1 AU is about 0.000016 light year. The nearest star, alpha Centauri, is over 4 light years distant, or about 280 000 AU.
The absolute magnitude of the Sun, the magnitude it would have at a distance of 10 parsec or 32.6 light years, is +4.8.
The Sun is often stated to be an "average" star. This is somewhat misleading. Among the nearer stars with an absolute magnitude higher than the Sun's are about 10 times more frequent than those with a lower absolute magitude. That is stars fainter than the Sun are far more frequent than stars brighter then the Sun. The Sun outshines many of the fainter stars by a factor of 1 000 to 10 000.
However nearly all the stars which can be seen with the naked eye are really brighter, many of them much brighter, than the Sun. Among the brightest stars as we see them, there is only one, alpha Centauri, with a comparable absolute magnitude to the Sun. All the rest are intrinsically far brighter.
With a magnitude -1.46, Sirius is the brightest star (after the Sun). The star is at a distance of 8.6 light years, 2.6 parsec. Its absolute magnitude is +1.4, so Sirius would be about 23 times as bright as the Sun if the two were at the same distance.
Sirius is a double star, with a relatively faint white dwarf companion. The companion is difficult to see due to the extreme brightness of Sirius.
Sirius is at its best evening position for viewing in the early months of the year. Having a southerly declination, it is high in southern skies. From the northern hemisphere it is a winter object, rather low the from mid latitudes.
Canopus is prominent high in the evening sky during the summer months in the Southern hemisphere. With a declination over 52°S it is a circumpolar object, never setting for all places south of about 40° south. For places in the northern hemisphere with a latitude north of about 37° the star never rises.
The diameter of Canopus is thought to be 30 times that of the Sun. However it is probably only a little over 10 times as massive as the Sun.
Due to its Southerly declination, α Centauri, sometimes known as Rigel Kent, is circumpolar for places in the Southern hemisphere with a latitude south of 30° S. It is the brighter of the two pointers.
α Centauri is a triple star system and the nearest star to the Sun. The naked eye star is in fact a pair of stars, at present separated by about 13" but this distance will decrease in the next few years. A small telescope shows them as a brilliant pair, although as they close they will get more difficult to separate. The brighter of the two, α1, is a similar star to the Sun in colour and size. It has a magnitude -0.01 and is about 1.5 times as luminous as the Sun, 1.1 times as massive and 1.07 times the Sun's diameter.
α2 Cen is a slightly cooler star, with a magnitude 1.35. It has only about 0.4 of the luminosity of the Sun and about 0.85 of its mass. However it is the bigger star, with a diameter 1.22 times that of the Sun.
The two stars orbit each other with a period about 80 years. The orbital ellipse has an eccentricity of 0.52, which makes it much more elongated than the ellipses of planets. The actual separation of the pair of stars varies between about 11 and 35 astronomical units, that is from just over the distance of Saturn from the Sun, to more than the distance of Neptune. From the Earth we see the orbital ellipse nearly edge on. As a result the ellipse appears to be very narrow, with the separation of the two stars varying between 22" (eg in 1980), down to about 2", as it will be in 2037.
Proxima Centauri, the third star of the system, is a faint star, magnitude 10.7 so not visible to the naked eye. It is very distant from from the bright pair. It is also slightly closer to the Earth, so is the closest star of all. If it does orbit α Centauri, its period must be hundreds of thousands of years. Proxima is an extremely faint star, the Sun is about 13000 times as luminous. The actual separation of Proxima and Alpha is about 10 000 Astronomical Units or two light months.
Arcturus in the brightest star north of the equator. With a declination +19.5°, it only reaches an altitude of about 30° as seen from New Zealand. The star is cooler and a little more yellow than the Sun. Its diameter is about 25 times that of the Sun, but its mass is only about 4 times that of the Sun, giving it a very low density, about 0.0003 the Sun's density.
Vega's declination of almost 39° north of the equator makes it a low star for southern hemisphere observers. From the extreme south of New Zealand it will only just rise above the northern horizon. On the other hand, Vega is a brilliant Summer star for northern hemisphere observers.
Vega is considerably hotter than the Sun with a surface temperature of about 9200 kelvin (about 9000°C). Its diameter and mass are both about 3 times that of the Sun.
With a northerly declination of 46°, Capella does not rise for places south of about Christchurch, latitude 43.5° south, in the southern hemisphere. It will be very low for places in New Zealand north of this. In the northern hemisphere it is a prominent winter star.
Capella is a close double star, the two stars appearing too close to separate with a telescope. The components orbit each other in 104 days: their distance apart is less than 1 astronomical unit (the distance of the Earth from the Sun). Both stars are considerably bigger than the Sun, with diameters about 13 and 7 times that of the Sun and masses respectively 3.0 and 2.8 times the Sun's.
Despite being designated β, Rigel is the brightest star in Orion. Being only 8° south of the equator, Rigel is visible from almost the whole Earth.
Intrinsically Rigel is one of the brightest stars known having an absolute magnitude -6.4. Its surface shines at a white hot temperature of about 12000 Kelvin, twice that of the Sun. If, instead of being some 680 light years from the Earth, Rigel was at the distance of the nearest star, αCen at 4.3 light years, Rigel would shine at magnitude -11. The full Moon is less than 2 magnitudes brighter.
Rigel is estimated to be about 50 times the mass of the Sun but it puts out about 30000 times as much energy as the Sun. Thus it is effectively putting out energy per unit mass 600 times as quickly. In view of this, the life expectency of Rigel should be one six hundredth that of the Sun perhaps about 20 million years. Hence the star can only be a few million years old, compared to something like 4.5 billion for the Sun.
Rigel has a magnitude 6.7 companion star, about 9" from it. A 15 cm telescope will show this, although the brightness of Rigel can make it difficult to see in small telescopes.
Like Rigel, Procyon is close to the equator, just over 5° north, so is also visible from all the Earth except close to the South Pole. In terms of its true brightness, Procyon is about 7 times brighter than the Sun with a surface temperature about 7000 Kelvin. Procyon's diameter is just over double and its mass is just under double that of the Sun.
Like Sirius, Procyon has a white dwarf companion, but being more than 10 magnitudes fainter than Procyon, it is not possible to see in a small telescope. Like Sirius' accompanying white dwarf, the presence of a companion of Procyon was first detected by irregularities in the proper motion of Procyon, which are as fairly large at 1.25" per year.
Achernar is another of several bright stars with a declination near 60° south, being at the southern end of the long constellation Eridanus. As a result it is not observable from much of the northern hemisphere, but is a circum-polar star for much of the southern hemisphere.
The star is a very hot star with a surface temperature close to 14000 Kelvin, even hotter than Rigel. However Achernar, with a diameter about 7 times that of the Sun, is not as large as Rigel, so is not burning itself out at quite such a prodigous rate.
At the opposite corner of Orion to Rigel, Betelgeuse is one of the best known "red giant" stars of the sky. Betelgeuse has a marked variability in magnitude, at its brightest it has occasionally out-shone Rigel, so perhaps justifying its designation as α Ori.
Betelegeuse is one of the largest stars known. It size varies enormously as its brightness changes, by as much as 60%. With a diameter estimated to range between 800 and 1300 million kilometres, that is 550 to 920 times that of the Sun, Betelgeuse would, if positioned at the Sun, extend well beyond the orbits of both the Earth and Mars. Despite this enormous size the star is only about 20 times as massive as the Sun. As a result the average density is very low, less than one ten-thousandth that of air. That is similar to a good vacuum!
As might be expected from its colour, the surface temperature of Betelgeuse is much lower than the Sun's, about 3100 Kelvin. This results in its orange-red colour - like an electric fire much of its radiation is given out in the infra-red. If our eyes were sensitive to Infra red radiation, Betelgeuse would be the brightest star in the sky.
Hadar, often just known as beta Centauri, is another of the declination 60°S bright stars and only 4.5° from α Cen. Hadar is the fainter of the two pointers.
Unlike its brighter partner, Hadar is far brighter than the Sun, with something like 1500 times its luminosity, but at a distance of about 490 light years is more than 100 times further from the solar system than is α Cen.
Hadar is also a double star, the companion having a magnitude 4.1, But being only 1.3" away from the primary, the two are much more difficult to separate.
Acrux, or alpha Crucis is the brightest star of the small kite shaped constellation forming the Southern Cross, the constellation pointed at by alpha and beta Centauri. Acrux is at the foot of the kite, being the most southerly of the four stars. Hence it is also one of the bright stars with a declination about 60° south, and essentially a southern hemisphere star.
Acrux is a similar star to beta Centauri but nearly twice as far away, so is actually more luminous. It is also a double star. With a separation between the pair of 4.4", it is easier to separate than β Cen although still needing a reasonable telscope and good seeing conditions to do so. The two stars are thought in reality to be about 500 Astronomical Units apart so the period will be several thousand years. They have magnitudes of about 1.3 and 1.6 respectively; the secondary should not be confused with a third star some 90" from the pair with a magnitude 4.9. This star can be separated from the pair with binoculars.
Altair is the brightest star Aquila a constellation stradling the equator. The star is the fourth closest of the bright stars at 16.8 light years. It has a very rapid rate of rotation , completing one turn in about 6� hours, with its equator moving at 250 km per second. As a result, Altair is thought to be very flattened with an equatorial diameter nearly twice the polar diameter. By comparison, the Sun rotates once in 25.4 days with an equatorial speed of 2 km per second.
Aldebaran is the red eye of the constellation Taurus, the Bull. It is a moderate sized red giant star about 40 imes the diameter of the Sun. Thus it is much smaller than Betelgeuse. Although it looks to be part of the Hyades cluster, Aldebaran is only half the distance of the group of stars forming the Hyades cluster.
As its colour suggests. Aldebaran is a cooler star than the Sun, with a surface temperature of about 3400K. Like other red giants it has an extremely low density, probably about .00005 of the Sun's density, and only one-twentieth of the density of the Earth's atmosphere at sea level.
Spica is a brilliant blue-white star, being about 2000 times as luminous as the Sun. In fact is is an extremely close double star, with a period of just over 4 days. The two form a grazing eclipsing system, that is the secondary passes in front of and behind the primary as seen from the Earth. The two stars are probably about 17.5 million kilometres apart, with the secondary about half the size of the primary. The system has a small magnitude range of about 0.07.
The name Antares means the "Rival of Mars", and comes from the conspicuou red colour of the star, similar to the colour of Mars. Antares is another red giant star of enormous size some 700 times the diameter of the Sun. Of the birght stars, it is only exceeded by Betelgeuse. With a radius of something like 500 million kilometers it would, like Betelegeuse extend beyond the orbit of Mars if positioned at the Sun.
Despite its huge size, Antares is only 10 to 15 times as massive as the Sun, resulting in its denisty being less than one-millionth that of the Sun.
Antares is a double star with a blue to green companion about 3 arc seconds from the primary. In good seeing conditions this can be seen as a emerald spot immersed in the ruddy glow of the primary red giant. In fact the secondary is about 50 times as luminous as the Sun. Antares itself is something like 10000 times as luminous as the Sun.
Although Pollux is labelled β (beta), it is the brightest in the constellation Gemini, outshining its companion "twin" by about one-third of a magnitude. With a distance close to 10 parsec, the standard distance for absolute magnitude, the apparent and absolute magnitude are nearly the same.
Deneb is the most distant of the brighter stars with a distance over 3000 light years, although there is a considerable error remaining in this figure. Taking this distance as correct, the absolute magnitude is -8.7, and so the star is intrinsically even brighter than Rigel. This makes it about 250000, a quarter of a million, times as bright as the Sun. A planet would have to be at a distance of 500 Astronomical Units from the star to receive energy at the same rate the Earth does from the Sun.
If Deneb was at the distance of the nearest star, Rigil Kent, it would have a magnitude about -13, so giving as much light as the full Moon. With its concentrated light, precautions, similar to those for observing the Sun, would have to be taken to view the star.
The mass of Deneb is probably about 25 times the Sun's making it one of the greatest super-giant stars known. However, because of its enormous output of energy, its life expectancy is only about one-ten-thousandth that of the Sun, so of the order of a million years. By stellar standards, Deneb cannot been around very long, and has only a short life span.
With a northerly declination of +45°, Deneb is a very low star as seen from New Zealand, and does not rise at all for the southern parts of the South Island.
The second brightest star of the Southern Cross, Mimosa, often just known as Bercux or beta Crux, is a rather similar star in colour to alpha. It is slightly further away than alpha, and like alpha, in reality about 3000 times as bright as the Sun.
Mimosa is a variable star, but with a light range of 0.06 magnitudes, this is not detectable by eye. The period is below 6 hours.
Regulus is the closest bright star to the ecliptic, only about half a degree from it. As a result it is, at times, occulted by the Moon and the planets also regularly pass close to it. At about 13000 K, the surface temprtature of Regulus is much higher than the Sun's and the star is considerblay whiter in colour.
Regulus has a small companion star, with a magnitude a 7.9. With a distance of 3' from Regulus the secondary is easy to see through a small telescope. The actual distance from Regulus is about 4500 Astronomical Units, so may well take about 300000 years to make one orbit.
Castor is the fainter of the twins even though its partner, Pollux, is labelled β (beta). The two stars form an obvious pair about 4.5° apart. They are rather low as seen from New Zealand.
Castor is itself a double star, the two components having magnitudes 1.9 and 2.9 being separated, 2004, by just over 4 arc seconds. This makes them a fine pair for a small telescope in good seeing conditions. Their period of revolution is about 400 years, the stars having a mean separation of about 90 astronomical units, three times the distance of Neptune from the Sun.
There is a third component, a 9.1 star some 72.5" from the pair. This probably takes more than 10000 years to make an orbit. In addition each of the three visible stars is itself a spectrocopic pair, ie has a very close companion detected by its characteristic spectral lines. The periods of these companions are: Castor A - 9.2 days, separation about 6 million kilometres, Castor B - 2.9 days, separation about 5 million kilometres, Castor C - 0.8 days, separation 2.5 million kilometres. This last pair orbit almost edge on as seen from the Earth ans so are an eclipsing pair.
Shaula is the brighter of the two stars forming the sting in the tail of the Scorpion. Its companion is the 2.65 magnitude star υ Sco, Lesath, which has a magnitude. The two are just over half a degree apart.
Both stars B type stars far hotter than the Sun. Shaula is has the greater luminosity probably about 10000 times the Sun's.
Gacrux is the third brightest star in the Southern Cross of which it is the top star. The star is a red giant similar to Antares but rather smaller. Gacrux is only some 140 times as luminous as the Sun, compared to Antares which is about 10000 times.
The star has a relatively large annual proper motion, just over a quarter of an arc-second per year in southerly direction.
The apparent brightness of a star is given in terms of its magnitude. Magnitude uses a logarithmic scale with larger numbers representing fainter stars. The northern hemisphere star, Vega, has a magnitude close to zero. Vega is the 5th brightest star in the sky (not including the Sun), so the 4 brighter stars have negative magnitudes. Thus Sirius, the brightest night time star, has a magnitude of -1.46. This means it is nearly 4 times as bright as Vega.
On the magnitude scale, if two stars differ by one magnitude, then the brighter star, with lower magnitude, is 2.512 times as bright as the fainter. A difference of 5 magnitudes represents a difference in brightness of exactly 100 times. (2.512 raised to the power 5 = 100). While the brightest stars visible to the naked eye have a magnitude near to 0, the fainter stars have magnitudes 5 or more.
The apparent brightness of a star depends on its distance, the further the star, the fainter it will appear. To compare the intrinsic brightness of stars, the magnitude the star would have at a distance of 10 parsec (32.6 light years) is determined. This is known as the absolute magnitude of the star. Thus the absolute magnitude of Vega is +0.5, whilst that of Sirius is +1.4. Thus if these two stars were both at the same distance, Vega would appear brighter, it produces about 2.3 times as much light as Sirius. At a distance of 10 parsec, the Sun would have a magnitude +4.8 (its absolute magnitude is +4.8). Thus at this distance the Sun would appear similar in brightness to the fainter stars visible to the naked eye.
The figure in this column is a conversion of the absolute magnitude into a figure comparing the true brightness of the star to the Sun. Thus Sirius, luminosity 23, is radiating about 23 times as much light as the Sun.
This relates to the surface temperature and hence the colour of the star. A sequence of letters is used: OBAFGKMQ with O being the hottest and most white star. The Sun is a G type star and is somewhat yellow. M type stars are much cooler than the Sun and are distinctly orange to red in colour. Antares and Betelgeuse are good examples of this type of star.
The distances to stars are usually given on light years. A light year is the distance travelled by light in 1 year, which is just under 9.5 trillion kilometers (9 500 000 000 000 km). Alternatively the distance is given in parsec, which is the distance at which the radius of the Earth's orbit round the Sun subtends an angle of 1 arc second. One parsec is about 3.26 light years or just over 30 trillion kilometres.
This is the date on which the star will be due north, or south, and at its highest in the sky at 21 hours local solar time, that is about 9 pm. Local solar time is likely to differ a little from clock time. In New Zealand, clock time in the winter months (NZST) corresponds to solar time at longitude 180°. Consequently, on the dates given, transits in much of NZ will be at about 21.30 (9.30 pm). When clocks are put forward in Summer months transits in NZ will be at about 10.30 pm on the date shown.
Each day a star will transit about 4 minutes earlier than the previous day, that is nearly half an hour earlier each week. After a month the star will transit 2 hours earlier, while in the previous month it would transit 2 hours later, and so on. The altitude of the star at transit depends on the latitude of the observer and the star's declination.
Right Ascension is measured in hours and relates to when stars will transit. If one star has an RA 1 hour higher than another, it will transit 1 hour later than the second star. As a rough rule of thumb, dividing the RA hour by 2 will give the month when the star will transit at about 20 hours, or 8 pm.
Declination for a star is equivalent to latitude for a place on the Earth. Positive (+) declinations are north of the equator and negative (-) are to the south. If a star has the same declination as your latitude, then it will pass immediately over head at transit.
Knowing the declination of a star gives an idea of how high it will be at transit. The closer the star's declination is to your latitude the higher it will be. To be more precise, the difference between your latitude and the star's declination gives the angle from the zenith (the point over your head) the star is at transit. If the difference is more than 90° the star will never rise at your position. If the difference is nearly 90°, the star will be very low to the north (from the southern hemisphere) or to the south (from the northern hemisphere) and will only be above the horizon for a few hours.
As an example, Christchurch, New Zealand has a latitude 43.5° south (-43.5°). Hence Canopus, declination -52.5° will be only 9° from directly overhead (to the south) when it transits. On the other hand, Vega declination +39° differs from the latitude -43.5° by 82.5° so will be 82.5° from the zenith at transit, that is only 7.5° above the horizon, so very low to the north.
ROYAL ASTRONOMICAL SOCIETY OF NEW ZEALAND