- In the early part of the 20th century, a classification scheme was
devised for stars based on their spectra. The scheme was originally based
only on the relative strengths of Hydrogen lines in the stars' atmosphere.
Type A stars had the strongest hydrogen lines, type O the weakest. The
different classes were then rearranged in order of decreasing surface
temperature. Some letters were rejected (e.g., C, D, E) due to redundancy.
- Today, stars are classified according to their surface temperature. Each
temperature range is known as a spectral type.
From hottest to coolest the order is: O B A F G K M
- Our Sun is a G star.
- Different star classes are characterized by the
prominence of certain spectral lines
- There are several mnemonics to remember this sequence
- Oh Be A Fine Girl/Guy Kiss Me (This is the most famous one)
- Only Boys Accepting Feminism Get Kissed Meaningfully
- Old Baboons Angrily Fling Green Kiwis and Mangos
- Only Bumbling Astronomers Forget Generally Known Mnemonics
- Only Bad Astronomers Feel Good Knowing Mnemonics
|Luminosity (solar luminosity)
Approximate main-sequence life span (years)
|| % of all
Main Sequence Stars
- Because two stars with the same surface temperature can be very different
(e.g., red supergiant vs red dwarf), an additional label, known as luminosity
class, is added to unambiguously describe a star.
- Luminosity class is designated with a Roman numeral and describes a stars
state of evolution. For example, a giant (class III) is more evolved than a
main-sequence star (class V).
- The full classification for our Sun is G2 V. The G2 spectral type means
it is yellow-white in color and the luminosity class V means it a
hydrogen-burning, main-sequence star.
- Betelgeuse is an M2 or a red supergiant.
- Proxima Centauri is an M5 V, similar in color and surface temperature to
Betelgeuse, but less evolved and far dimmer because of its far smaller size.
Class of Star
wd or D
Hertzsprung-Russel (HR) diagram
- When stars are plotted on a luminosity vs surface temperature diagram (HR diagram),
several interesting patterns emerge:
- Most stars fall on the Main Sequence.
- On the Main Sequence, the more massive stars are bigger, hotter,
more luminous, and die faster.
- The life span of stars ranges from about 10 million years for the
blue giants to about 100 billion years for the red dwarfs.
- The most common type of star is the red dwarf (lower right); the
least common type is the blue giant (upper left).
- This classification was originally proposed in 1912 by astronomers Ejnar Hurtzprung and
Henry Norris Russell, hence the designation HR diagram.
- Luminosity of stars if often expressed in units of the Sun's luminosity
(L = 3.9 x 1026
- The HR diagram spans a rather large
range in luminosity, from 10-4L
on the low end to as much as 106L
on the high end.
- This interactive
applet might help you visualize some of the properties of the HR
- Of the 12 brightest stars in our sky,
most are giants and supergiants.
- Our Sun is a main-sequence star dwarfed by a supergiant like
- Star mass ranges from 0.08xMsun to 100xMsun:
- Stars more massive than about 100xMsun release too much energy through
nuclear fusion and are unstable.
- Stars less massive than 0.08Msun are too small to sustain
nuclear fusion. Very large objects below this limit are sometimes called
- The lifetime of a star is directly proportional to the amount of fuel it
has (i.e., mass) and inversely proportional by the rate at which it burns
the fuel (i.e., luminosity). Putting these together, we can estimate the
lifetime t to be proportional to M/L.
- Empirically, for stars on the main sequence, luminosity is roughly
propotional to the cube of the mass (L ~ M^3). Consequently, plugging this
in for L, we find that the lifetime is inversely proportional to the square
of the mass (t~1/M^2).
- Since the most massive stars are about 100 times the mass of the Sun,
their lifetimes must be about 1,000 times shorter, or about 10^6
(10^10/10^4)years. This is indeed what we see; the most masive stars burn
out in about 4 million years.
- Similarly, the least massive stars are about one tenth of a solar mass.
They survive about 100 times longer than our Sun, or about 10^12 years.
Because this is well over ten times the present age of the universe, none of
these smaller stars have died yet.
- Many stars are found in one of two types of clusters: open and globular.
- A famous star cluster visible to the naked eye is the Pleiades, also
known as the Seven Sisters.
- Our Solar System is not part of a star cluster.
- The HR diagram is generated through a careful study of star clusters.
Clusters are important because:
- All the stars in a cluster lie at about the same distance from
- All the stars in a cluster formed at about the same time.
- The age of a cluster corresponds to the main
sequence turnoff point.
- Stars with life spans equal to this age are exiting the main sequence.
- Smaller stars with life spans longer than this age are still on the main
- Larger stars with life spans shorter than this age are off the main
- Open clusters are or are characterized by:
- Few stars (10-1000)
- Relatively small (about 30 ly)
- Always found in the galactic disk
- Relatively young (less than 7 BY)
- Enriched in the heavy elements
- Globular clusters are or are characterized by:
- Lots of stars (104-106)
- Relatively large (~50-150 ly)
- Found mostly in halo
- Relatively old (12-16 BY)
- Virtually no heavy elements
Summary of stellar properties
||How it is determined
||-Compare brightness in two different E&M spectrum bands.
-Examine spectra of star
||Use color or spectra
||-Directly measured via parallax
-Indirectly measured via method of standard candles
||-use brightness and distance
-Eclipsing Binary: Use Temperature and Size
||Doppler shift of spectral lines
||proper motion and distance
|Rate of Spin
||Doppler broadening of Spectral Lines
||-Luminosity and Temperature
-directly measure in an Eclipsing Binary
||-Use Kepler's 3rd Law in Binary System.
-Infer from Luminosity and Temperature.
-Infer from spectral lines.
|Strength of Magnetic Field
||Main-Sequence Turn-off Point in a cluster
Return to class notes TOC.
Page last modified: