For professional astronomers, magnitudes might be viewed as a 'burden of history' (a more scientific measurement of a star's output would be in terms of Watts). But for non-professionals they are very convenient. They allow the stars to be classified in terms of how bright they actually appear to the human eye. The stars were classified by the ancients (more specifically, there are records by Hipparchus from 150 BC) into six levels of magnitude, the first magnitude being the brightest and sixth the dimmest. In the whole sky, there are about 6,000 stars capable of being detected by the unaided eye.
Although these magnitudes appear logical to the human eye there is biological effect operating here which is not operating in line with the 'real' scale. A better known analogy concerns sound. A level of 6 decibels is 10 times as powerful as 5 decibels, but 5 decibels is 10 times as powerful as four decibels, so 6 decibels is actually 100 times as powerful as four decibels. The human ear will not detect this 'real' scale, but will nevertheless consider the decibel scale to be 'logical'.
The crude classification of Hipparchus had been extended into an even more unreliable scale when fainter stars were being discovered by telescope, different observers sometimes assigning magnitudes for the same star differing by as amny as five magnitudes. About 1850, the British astronomer N. Pogson had established a convention, which was adopted by E.C. Pickering of the Harvard Obseravtory and has become the standard. Under this system a difference of one magniude corresponds to a ratio of light intensity equal to the fifth root of 100 (2.512). In other words, a star of a particular magnitude is 'really' 2.5 times as bright as a star of one lower magnitude (when measured in Watts). Using this 'real' scale, a first magnitude star is 2.55 times more powerful than a sixth magnitude star , i.e. it is 100 times as bright.
The 'real' scale, as I have called it, is referred to as luminosity. Since it is measured in Watts, you can see it is really a measure of power.
The ancient classification of magnitude has been modified so that we can now talk about magnitudes in decimals, not just whole numbers. A few bright stars have been re-classified to have negative numbers (as explained already, the lower the magnitude, the brighter the star is). By extension the Sun has a negative magnitude (-26.7). The full moon is about -12.5 and Venus at brightest is about -4.
Limits of Observation
|15 cm telescope||13|
|5 meter telescope||20|
|Most powerful telescopes||24|
So far we have been talking about apparent magnitude . The absolute magnitude . is the magnitude that a star would have if viewed at a standard distance of 10 parsec.
Apparent magnitude and absolute magnitude are related by the following formula
where M is absolute magnitude, m is apparent magnitude, d is the distance of the star and log denotes logarithm (to the base 10)
a) Star α has a magnitude of 3.0. Star δ has a magnitude of 5. How mant times brighter is α than δ ?
Twenty-Five Brightest Stars
|Star||Constellation||Type||Absolute Mag.||Distance (LY)|
The first four have 'negative' apparent magnitudes.
- Sirius -1.46
- Canopus -0.7
- Alpha Centauri -0.3
- Arcturus -0.04
- Canopus -0.7
The first 21 stars, down to Regulus (1.35) are classed as first-magnitude.
The vertical axis depicts the inherent brightness of a star, often in either of two equivalent ways
- The luminosity (measured in Watts) this measure is NOT shown on the above H-R diagram
- Absolute Magnitude
The horizontal axis is graduated differently to convention and increases from right to left. It represents either of two equivalent measures
- The temperature (here shown in Centigrade but also often shown in Kelvin, i.e. Centigrade (or Celsius) plus 273)
- Spectral Class, ranging across (hottest first) O, B, A, F, G, K, M, R, N, S (Oh Be A Fine Girl Kiss Me Right Now Sweetie)
About 90% of stars fall on the Main Sequence. Originally it was thought that maybe stars started off on the top left of the Main-Sequence and descended down the Sequence as they aged. Nowadays it is known that a star enters the Main Sequence when it starts to burn hydrogen in its core. It enters at a definite place on the Main Sequence and more or less stays at the same location during its entire hydrogen-burning phase.
Note that as you ascend the Main sequence, the mass of the stars increase. This increased mass will mean more hydrogen is being burnt, producing a brighter star, which then means that higher mass stars will have a shorter lifetime on the Main Sequence than less massive stars. This effect is accelerated by the more efficent nuclear reactions that the more massive stars are able to generate.
This particular characteristic can be used to calculate the age of a cluster - a Hertzsprung-Russell diagram for the stars in a cluster will show a definite upper limit for Main Sequence stars (the turn-off point), all stars more massive than this will have ended their hydrogen burning existence already.
And following on from what we have just said, smaller stars are much more numerous than other types of stars.
You can see that the Sun is a G star. These stars have lifetimes of around 10 billion years - the Sun is about halfway thru this lifetime.
Each spectrum class can be broken down into 10 divisions - the Sun appears to be G2, although different values are quoted.
The Hertzsprung Russell Diagram seems to show that the maximum mass for a Main Sequence is about 60 times the Solar Mass. The minimum mass of a star is about 0.1 of a Solar Mass, the mass required to produce nuclear reactions in the core. The surface temperature of stars varies from 2000 to 35,000 degrees.
Population I and Population II Stars
In the 1940s, Walter Baade classified stars into two populations. The naming of these populations appear to be the 'wrong way round' given that they actually concern differences related to age.
Population I stars are younger stars with a relatively high metal content (about 1% by mass).
Population II stars are older. They formed at the time when the Universe was primarily Hydrogen and Helium - it had not been significantly seeded with heavier elements by Supernova explosions.
The difference in metal content, although small, makes a significant difference to the way that stars evolve. To be more precise, it makes a significant difference to the size of stars that are formed - clouds contracting to form population I stars tend to fragment more and proceed to produce smaller stars.
Population II stars predominate in elliptical galaxies and in the center of spiral galaxies i.e. in the bulge and in globular clusters orbiting predominantly in the halo. Population I stars predominate in the spiral arms of spiral galaxies.
Many of the bright stars have names derived from Arabic and also Greek.
In 1603, Johann Bayer published his Uranometria star atlas which listed stars using letters of the Greek alphabet. In general, the sequence was stated in terms of decreasing magnitude although there are discrepancies, for example α Orionis is Betelgeuse and β Orionis is Rigel, despite Rigel being brighter than Betelgeuse.
In 1725, Flamsteed's catalog Historia Coelestis Britannica appeared which introduced a numerical system, with the sequence running west to east. Under this system Orion is 58 Orionis. This naming also covered fainter stars than Bayer's
About half of 'observed stars' are binary stars - so actually two-thirds of stars are members of a binary system (if you're still with me).
It was William Herschel in 1802 who first produced a proof that binary systems existed. John Michell had predicted them mathematically in 1767 - he found that the incidence of apparently close pairings of stars was too great for them all to be effects of line of sight.
Some of the better know binaries are
Alpha Centauri Furthermore, Proxima Centauri rotates around this double making it a multiple star system
Mizar in the Plough, was the first binary to be discovered in 1650, although it wasn't until the work of Herschel that it was recognized as such.
Albireo in Cygnus. One is bright yellow (3.1) and the other dimmer and bluish (5.1).
Sirius Sirius A actually has a White Dwarf as a companion. Although Sirius is very close to us, Sirius B is very hard to detect.
Algol is an eclipsing binary. Its brightness varies in a regular way as one binary blocks off the light from the other. Every 69 hours it dims from second to third magnitude, as was already known to the Arabs before the Middle Ages. The pair cannot be resolved by telescopes on Earth. It lies in Perseus, marking the head of Medusa. It was John Goodricke in 1782 who realized that it was acting in such a strange way because it was a binary. Its light curvve is shown below.
γ-Andromedae consisting of an orange component (mag. 2.2) and a blue component (mag. 5.0).
Spectroscopic Binaries is the name applied to binaries whose nature can only be detected via their Doppler Shift.
Mira (Omicron Ceti) was the first star recognized to be a variable star, in 1596 (by Fabricius). Fabricius actually actually noticed it as a new star, then it disappeared, reappearing in 1609. It is capable of varying in 11 months between second magnitude and tenth magnitude, although the range of variation varies. As is common, a variable star gives its name to a class of stars, if it is found that other stars also behave in the same way - which is the case here. All Mira stars are Red Giants and evidence points to them being stars dying thru pulsations, these pulsations gradually causing the star to lose matter. They are also called long-period variables.
Flare Stars are red dwarfs that flare up briefly for a few minutes and can take only a short time to flare up, e.g. 20 seconds to change by six magnitudes. This activity is unpredictable. Proxima Centauri is a flare star.
A Nova is assumed to be caused by a White Dwarf in a binary star accumulating matter streaming from its companion, resulting in short-lived fusion and an increase in brightness by about 1000 times in a few days followed by a slower decline to original brightness. All novas observed are in binary systems anyway. Related classes are recurrent novae - less powerful but repeated over decades or centuries (only a couple of which ever reached naked-eye visibility), and dwarf novae (also called U Geminorum or SS Cygni stars) which are even smaller outbursts but recur on a time scale of months. These groups are also called cataclysmic variables.
Cepheids see Distance Ladder - Cepheids
W Virginis stars are actually Population II Cepheids. The identication of the different properties between Population II and Population I Cepheids led to a rescaling of the Universe - see Distance Ladder.
Red Supergiants Betelegeuse is a well known variable, see below. Mu Cephei is another, called the Garnet Star. This latter star is actually much more luminous intrinsically than Betelegeuse but much further away. It varies in magnitude between 3.4 and 5.1, with no well-marked period.
The first variable to be discovered in a constellation is given the designation R, the second S and so on until Z. The next variable is called RR, then RS until RZ. Next comes SS to SZ, TT to TZ, etc. etc. until ZZ. Next after that becomes AA - and so until AZ, followed by BB - up until BZ, followed by CC etc. etc. etc, leaving out J, up to QZ. This highly logical system allows 334 stars to be named. Any stars after this are designated with a system just using numbers.
Luminous solids, or liquids and gas under high-pressure will emit a continuous spectrum of all wavelengths. This is the type of spectrum that the Sun will emit allowing rainbow effects when its light is passed thru a prism.
On the other hand, low pressure gas (and this includes gases of what are normally solids or liquids under normal Earth conditions) will emit radiation at discrete values. In other words, it will emit radiation of certain fixed and distinct frequencies. This spectrum will be the signature of a particular substance - different substances have different spectra.
In addition to emitting only at fixed frequencies, a substance will also absorb radiation at the very same frequencies. This is an important feature in astronomy because this behavior will produce dark absorption lines for a substance of the same pattern as the emission lines produced by the very same substance.
Most famously, absorption lines of Sodium can be detected easily in the Sun's spectrum -
Fraunhofer Lines. These lines appear dark because they absorb light
from the Sun at a particular frequency, later emitting light of the same frequency except that
this emission will be in a different direction and so will not be detected by us - we notice
a dark line in the spectrum.
And the faint star in the Alpha Centauri system, Proxima Centauri, which is currently the closest star to us, is also a Red Dwarf.
Barnard's Star is a Red Dwarf and was detected in 1916, by E.E. Barnard. It is the star with the largest proper motion. This star is 6 light years away and lies in the constellation of Ophiuchis Its surface temperature is 3000 degrees. Its movement is 10.29 seconds of arc per year - it takes 180 years to move distance roughly equal to full moon.
Wolf 359 in Leo is a Red Dwarf. For a long time it held the record as the least intrinsically luminous star known. Only Alpha Centauri and Barnard's Star lie closer to us.
These stars are not actually brown - they emit a feeble glow of infra-red.
For a period at least, they were strong candidates for the 'missing' dark matter in the Universe.