The Cosmic Rosetta Stone

Dateline 1800 (or there-about): Napoleon's troops, while snooping about Egypt discover, at Rosetta, a stone on which appear hieroglyphics as well as Greek and other ancient languages.

Dateline 1800 (or there-about): British troops drive back the French. Found left behind is a stone slab on which appear hieroglyphics as well as Greek and other ancient languages.

Dateline 1800 (or there-about): Egyptologists at the Royal Museum in London announce that the Rosetta stone (named after the stone returned (stolen?) from Egypt is a translation table that allows translation of hitherto unknown languages. This is a crucial discovery for archaeology!

Dateline 1910 (or there-about): The Danish astronomer Eijnar Hertzsprung and the American astronomer Henry Norris Russell announce the discovery of a cosmic Rosetta stone. A strong correlation exists between stellar type and luminosity (equivalent to absolute magnitude)

Spectral Classification

By 1900 it was clear that the myriad of stars revealed by telescopes and the application of photography could be fit into families of shared spectral characteristics. This led to the development of the spectral sequence OBAFGKM (the Harvard system)

Spectral Type	Spectral Features	Temperature (K)
O	HeII emission	28 000 - 50 000
B	HeI lines	9 900 - 28 000
A	H lines	7 400 - 9 900
F	metals, H	6 000 - 7 000
G	Ca II, metals	4 900 - 6 000
K	Ca II, Ca I, molecules	3 500 - 4 900
M	molecules, C molecules	2 000 - 3 500

see all types on same page

The Hertzsprung-Russell Diagram

In the last lecture we saw that the distance-modulus formula gave us a "level-playing-field". By factoring out the effects of distance and turning magnitudes into absolute magnitudes ("10 parsecs away view") we can compare the actual brightnesses of stars. We also recall from an even earlier lecture that stellar brightness is also strongly correlated with surface temperature. If we plot absolute magnitude and stellar temperature we get the following:

The HR diagram as it is called truly is a Rosetta stone for astronomers. By looking at a star's position on the HR diagram, a mere glance tells a great deal about that star. The discovery of the HR diagram ranks as one of the most significant discoveries in astronomy since Kepler. One of the truly great discoveries of science is that the HR diagram is not a static diagram but instead is a dynamic one. Understanding this dynamic will be our main concern for the rest of this term.

Features of the HR Diagram: Luminosity Classes

A detailed matching of absolute magnitude with spectral type reveals that the stars fall into roughly 7 locii or patterns. These are called the luminosity classes. Stars of apparently the same or similar spectral type can at times have significantly different luminosity.

The Luminosity Classification is as follows:

Letter Designation	Luminosity Class
VII:	White dwarfs; very faint, extremely small but at times very hot
VI	Subdwarfs; similar to main sequence but somewhat smaller and less luminous
V	Main sequence; diagonal band stretching across the HR diagram.
IV	Subgiants; brighter than the main sequence, somewhat larger stars
III	Giants; very large and luminous stars
II	Bright giants; somewhat larger and brighter giants
Ib, Ia, IO	Supergiant stars; exceedingly bright and massive stars.

Using the HR Diagram - Spectroscopic Parallax

As we proceed in the course we will discover a number of uses for the HR diagram. Some of the key uses of the HR diagram are:

estimating stellar distances
measuring stellar age
estimating stellar lifetimes

In the following we will use the idea of spectroscopic parallax. This technique uses the HR diagram to unite two separate ideas:

spectral type relating to temperature
distance modulus

To see how this works consider the following scenario:

Example:
Through the telescope you find a faint blue star. Spectroscopic examination reveals that it is a B5 V star and your CCD photometer tells you that it has an apparent magnitude of 6.7. How far away is the star?

Solution:

The first thing to do is to look at the HR diagram. Since a B5 V star has a surface temperature of 15 500 K we can judge that its absolute magnitude is -1.1. We now know the distance modulus:

m - M = 6.7 - (-1.1) = 7.8

If you look at the distance modulus graph from the previous lecture you see that an object with this modulus must be about 360 pcs away or about 1200 ly away.

Seeds Chp 9; p. 178-188

Kaufmann Chp 19; p. 336-363