(Parenthetic note: Once you know T and L, you also know R (see
the notes for lecture 6). Thus, at every point on the H-R
diagram, you can compute the corresponding stellar radius.) One thing scientists like to do is devise classification schemes. (For instance, electromagnetic radiation is classified by wavelength; elements are classified by the number of protons in their nucleus.)
One way of classifying stars is by TEMPERATURE.
Alternate way of describing temperature: SPECTRAL CLASS
The spectral classes OBAFGKM began as a purely empirical method of classifying stars according to the appearance of the absorption lines in their spectra. Around 1900, Annie Jump Cannon realized that the strength of absorption lines depends on temperature. O stars are the hottest; M stars are the coolest.
Useful mnemonic: Oh Be A Fine Girl, Kiss Me.
(If you prefer: Oh Be A Fine Guy, Kiss Me.)
Temperature doesn't tell the whole story.
For instance, Betelgeuse and Proxima Centauri have the same surface temperature (they are both M stars). However, Betelgeuse is 300 million times more luminous.
Another way of classifying stars is by LUMINOSITY.
Just prior to WWI, Ejnar Hertzsprung (Denmark, 1911) and Henry Norris Russell (USA, 1913) separately had the same idea - classify the stars according to BOTH their temperature AND their luminosity.
Make a plot - temperature on the
horizontal axis (hot stars to the left; cool stars to the right)
and luminosity on the vertical axis (dim stars at the bottom;
bright stars at the top). This plot is called the
Hertzsprung-Russell diagram (or H-R diagram, for short) after its
inventors. An example of a Hertzsprung-Russell diagram is given
below.
(Parenthetic note: Once you know T and L, you also know R (see
the notes for lecture 6). Thus, at every point on the H-R
diagram, you can compute the corresponding stellar radius.)
Stars are NOT randomly distributed across the H-R diagram.
About 90% of all stars are on a narrow diagonal band running from the upper left corner of the H-R diagram (hot, luminous stars) to the lower right corner (cool, dim stars). This diagonal band is called the MAIN SEQUENCE.
The Sun is on the main sequence.
N.B. The above statements are true ONLY for Main Sequence stars. The remaining 10% of stars don't follow the main sequence path.
GIANTS are more luminous than a main sequence star of the same temperature. Therefore, they must be LARGER than a main sequence star of the same temperature. (That's why they're called giants.)
Properties of giants:
SUPERGIANTS are even more luminous than giants.
Properties of supergiants:
Supergiants have a wide range of temperatures; their distinguishing characteristic is that they are VERY LUMINOUS.
WHITE DWARFS are less luminous than a main sequence star of the same temperature. Therefore, they must be SMALLER than a main sequence star of the same temperature. (That's why they're called dwarfs.) They are called WHITE dwarfs because they are all fairly hot -- white hot, in fact.
Properties of white dwarfs:
The distinguishing characteristic of white dwarfs is that they are hot but dim.
Take a random sample of 1,000,000 stars from our galaxy. In this sample, you will find, on average:
(Numbers don't add to 1,000,000 exactly because they've been rounded off.)
Although supergiants are extremely rare, we know of their existence because they are extremely luminous, and thus can be seen for great distances.
A star as luminous as the Sun can be seen by the naked eye if it's closer than 17 parsecs (55 light years). An M main sequence star, with a luminosity one ten-thousandth that of the Sun, can only be seen by the naked eye if it's closer than 0.17 parsec (0.55 light years). By contrast, a supergiant star, with a luminosity 100,000 times that of the Sun, can be seen by the naked eye as long as it's closer than 5400 parsecs (18,000 light years).
