29:50 Modern Astronomy
Fall 2002
Lecture 27 ...November 18, 2002
Our Galaxy and Other Galaxies

(1) We live in a big, disk-shaped system of stars.
front and side views of Milky Way. The distance from the Sun to the center of the Milky Way is 8.5 kiloparsecs.

(2) This disk consists of a swarm of stars, together with gas that has not yet formed stars.

(3) The distance to the galactic center was traditionally measured via the use of stars called Cepheid variables. An example is the star Cephei. Cepheids show periodic variations in brightness, with the periods ranging from a few days to as long as 100 days.

(4) The big development in the story of the Cepheids came in the 1920's, when it was discovered that Cepheids have a Period-Luminosity Relation. See the discussion on this in the book, pp445-446. It was found that there is a relation between the period of the variation, and the absolute magnitude (or equivalently, luminosity) of the star.

(5) To determine the distance to a Cepheid, you

• Observe the star and measure the period of its variations.
• Infer the absolute magnitude from the period-luminosity relation.
• Measure the apparent magnitude (that part is easy)
• Calculate the distance modulus m - M and that gives you the distance.

(6) If you observe a Cepheid in an object (globular star cluster, galaxy, etc) and you determine the distance to the Cepheid, you have the distance to that object.

(7) There are Cepheids in globular star clusters, so we can measure the 3D distribution of globular star clusters in space. We assume that the center of the system of globular clusters is the same as the center of the Milky Way. That distance turns out to be 8.5 kiloparsecs. We will hear more about Cepheids later.

(8) After having assembled our ``model'' of the Milky Way galaxy, we can carry out a ``reality check'' by seeing if there are other objects like it out in space (much further than any of the stars you see in the night sky). We do see those objects. They are referred to as spiral galaxies, thus establishing the class of objects to which the Milky Way belongs. Pictures of galaxies are given below
NGC 253
NGC 891
the Sombrero galaxy
M51
Don't lose sight of the fact that these systems are about 20,000 parsecs in diameter!.

(9) Now for something very important; the stars in galaxies are not just suspended or stationary in space. Each star moves on an orbit through space determined by the gravitational force from all of the other stars. Most of these orbits (at least for stars in the disks of spiral galaxies) are approximately circular.

(10) Newton's theory of gravity shows that the characteristics of the orbit are again described by the circular orbit equation, but this time the mass M is the mass inside the orbit of radius R. So... if the Sun (or any other star) moves in a circular orbit in which the radius (distance from the galactic center) is R, and it moves with a speed V, then the mass interior to that orbit is related to R and V by

(11) We can use this formula to come up with something new and interesting: the mass of the Milky Way. For the orbit of the Sun about the galactic center, we have V = 220 km/sec and R=8.5 kiloparsecs. When we plug these into equation (1) we find that the mass interior to the orbit of the Sun is solar masses.

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