Lecture 5: Measuring the Milky Way

1
Lecture 5 Measuring the Milky Way

• Astronomy 5 The Formation and Evolution of the
  Universe
• Sandra M. Faber
• Spring Quarter 2007
• UC Santa Cruz

2
Longer-period Cepheid variables are brighter
Note another funny plot in which each tickmark
is the same FACTOR.
4
This kind of plot is called a log-log plot
because each tickmark is one step in the
logarithm.
3
2
3
The nearby globular cluster Messier 5
4
Circling around a globular cluster
5
Globular clusters are spherical because their
orbits are scrambled
6
The simplest nuclear reaction that makes stars
shine
7
Globular clusters in the spheroid of the
Sombrero galaxy
8
Milky Way in visible light (0.5 microns) stars
are badly obscured by interstellar dust clouds
9
Visible light is 350-700 nm, or 0.35-0.7 microns
Visible light averages around 500 nm, which is
0.5 microns (?m). One micron is one-thousandth
of a millimeter.
10
Near-infrared light lies at 1-3 microns,
between visible light and infrared (i.e., heat
radiation)
Near-infrared has slightly longer wavelengths
than visible light. Lies between 1-3 microns,
part way towards heat radiation, which is
called infrared.
HEAT
11
Milky Way at 1-3 microns stars seen through dust
Milky Way in visible light (0.5 microns) stars
obscured by dust clouds
12
21 cm radiation is in the short-wavelength radio
region
21 cm is a special wavelength that is emitted by
clouds of neutral hydrogen gas (H I).
HEAT
13
The Very Large Array of radio telescopes, which
observe 21 cm radiation. The VLA can cover up to
27 km and is located in New Mexico.
14
The Very Large Array of radio telescopes, which
observe 21 cm radiation. The VLA can cover up to
27 km and is located in New Mexico.
15
21 cm wavelengths (radio) reveal the hydrogen gas
layer in the disk. This layer fuels star
formation.
16
Major structural components of the Milky Way
17
A Milky Way-like external galaxy seen edge on
bulge
disk
NGC 891
18
The Sombrero is similar, but its spheroid is
relatively bigger
19
The orbits of spheroid stars in the Milky Way are
scrambled like those in a globular cluster
Spheroidal systems have scrambled orbits.
Disk systems have orderly orbits marching in
circles.
20
Map of hydrogen gas made with 21 cm radio
telescopes
Gas has density concentrations that look like
spiral arms
21
Stars form from dense clouds of gas
Giant H II region in Messier 33
Messier 33 galaxy, a nearby member of the Local
Group
22
The simplest nuclear reaction that makes stars
shine
Blue is clouds of hydrogen gas in Messier 33. H
II regions, where stars are forming, are red.
Notice how they line up.
23
Three views of the nearby spiral Messier 83
Visible light shows stars of all ages. Blue are
massive, youngest, most recently formed. Found
only in disk.
Near-infrared minimizes blue stars and maximizes
cooler, older stars, which populate both disk and
bulge.
21 cm shows hydrogen gas arranged in spiral arms.
This is where most stars ar forming.
24
Schematic explanation of long-lived spiral arms
The naturally circular disk orbits are deformed
by the gravity of the spiral arms in to ellipses.
Successively larger ellipses are rotated
slightly with respect to smaller ones. Even
though the stars (hardly) change speed as they
rotate around the center, their orbits converge
where the ellipses nearly touch. This spiral
pattern is what is needed to deform the ellipses
in the first place, and so the pattern is
self-sustaining.