Structure of the Milky Way

Last: 11. The Origin of Elements Next: 13. The Realm of the Nebulae



  15.1 Our Galaxy: The Milky Way   p. 345-346
  15.2 The Illusion That We Are at the Center   p. 346-347
  15.4 The Parts of Our Galaxy   p. 349-352
  11.8a,b Clusters of Stars   p. 267-271

The Milky Way from Mauna Kea

Photo of the Milky Way
Astronomy Picture of the Day [NASA]

All-Sky Views: Interpretation

Showing the entire sky on a flat map is tricky -- there's always some distortion. Most images of the entire sky use a projection similar to the one used here to map the entire surface of the Earth.   Mollweid projection of Earth
Map Projection Overview [Peter Dana]

All-Sky View: Visible Light

Photograph of entire sky
Astronomy Picture of the Day [NASA]

Distances in the Milky Way

To map the Milky Way we need to measure distances far beyond the reach of parallax. We use the inverse-square law: the apparent brightness B of a standard candle of known luminosity is inversely proportional to the square of its distance D.   The inverse-square law

Suppose we observe two stars and know that they have the same luminosity. Star 1 has brightness B1 at distance D1, and star 2 has brightness B2 at distance D2; then

D2 : D1 = √B1 : √B2

Another way to express the same relationship is

D2  =  D1  ×  √(B1 ⁄ B2)

Main-Sequence Fitting

How can we be sure that two stars have the same luminosity? One way is to use main sequence stars. Recall that the main sequence is a one-to-one relationship between a star's surface temperature T and its luminosity L, so any two stars with the same T must have the same L -- if both are on the main sequence!   Temperature-luminosity diagram

This technique works best if you happen to have a number of stars at the same unknown distance; that way, you can check that all the stars give the same answer for the distance you want to find. Luckily, nature makes stars in clusters...

Star Clusters

Whan a giant molecular cloud forms stars, it usually forms many stars at once. After the gas is used up, the stars may be left close enough to be held together by gravity, forming a star cluster.

It's possible that most stars, including the Sun, originally formed in clusters and later escaped.

  M11: the Wild Duck cluster
Astronomy Picture of the Day [NASA]

Star Cluster Dynamics

Orbital motion of stars in a small cluster. Color indicates stellar mass; red is low-mass, blue is high-mass. Notice that the high-mass stars tend to collect in the center of the cluster, while smaller stars are pushed to the fringes or even ejected.   Simulation of a star cluster

Distances to Star Clusters. I

One star cluster, the Haydes, is close enough that parallax yields a reliable distance of 46.3 pc. This provides a key step in the cosmic distance scale.   The Hyades cluster
The Hyades, Melotte 25 [SEDS]
Let's see how main sequence fitting can be used to find a distance to another nearby cluster, the Pleiades.   The Pleiades cluster
The Pleiades (star cluster) [Wikipedia]

Distances to Star Clusters. II

When we plot surface temperature T against apparent brightness B for stars in the Hyades and the Pleiades, we find that both clusters contain stars on the main sequence. However, the main sequence in the Hyades appears about 8.5 times brighter than the main sequence in the Pleiades:

BH = 8.5 × BP

  Temperature-brightness diagram

Using the inverse-square law, we can find the distance to the Pleiades:

DP  =  DH  ×  √(BH ⁄ BP)  =  46.3 pc  ×  √(8.5)  =  115 pc

Distances to Globular Clusters

Globular clusters are compact clusters containing hundreds of thousands to millions of stars. Their distances were measured using similar methods; they turned out to be many thousands of parsecs away.

These distances finally provided a sense of scale for the Milky Way.

The omega Cen cluster
Astronomy Picture of the Day [NASA]

Discovery of the Galaxy

Edge-on diagram of the Milky Way

Plotting positions of globular clusters showed that they form a swarm centered not on the Sun but rather on a point about 8500 pc = 8.5 kpc away. Once this was understood to be the center of the Milky Way galaxy, the big picture became clear.

All-Sky View: Infrared

IR image of entire sky
Astronomy Picture of the Day [NASA]

Stellar Populations

The stars making up the Milky Way can be divided into different populations on the basis of age. For this purpose, star clusters again provided an essential clue: since all the stars in a cluster form at almost the same time, the age of a cluster is equal to the lifetime of the most short-lived (massive) stars still on the main sequence.

Comparison of the main sequences in the Pleiades and the Hyades shows how this works. The main sequence of the Pleiades extends to hotter, more luminous stars because the Pleiades are younger. In fact, the Pleiades are only 115 Myr old, while the Hyades are about 625 Myr old.   Temperature-brightness diagram

(Note: 1 Myr = 106 yr, and 1 Gyr = 109 yr.)

Populations of the Milky Way

Population Location Age
(heavy elements)
I Disk of MW 0 to 10 Like Sun Flat, roughly
II Bulge and
Halo of MW
10 to 12 Much less
than Sun
Tilted, not

The idea of stellar populations helps to organize many aspects of the Milky Way's structure, and points toward an account of its history.   orbits in Milky Way
Last: 11. The Origin of Elements Next: 13. The Realm of the Nebulae

Joshua E. Barnes (
Last modified: November 9, 2006
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