Download Milky Way

Reminders!
Website: http://starsarestellar.blogspot.com/
Lectures 1-15 are available for download as study
aids.
Reading: You should have Chapters 1-14 read.
Read Chapters 15-17 by the end of the week.
Homework: Homework #4 is due tomorrow, June
24th.
Observing Projects: Both due Monday. They will
NOT be accepted late!!!
Our Home, the Milky Way
Today’s Lecture:
GRB Review (Chapter 14, pages 337-339)
• Two types of gamma-ray bursts
• Universal “central engine”
The Milk Way (Chapter 15, pages 344-365)
• The main parts of our Galaxy
• Nebulae
• What the spiral arms?
GRBs: Fantastic explosions!
• In 1991 new telescopes showed GRBs are isotropic on
the sky, proving they are (usually) NOT from our galaxy
• Afterglows (optical) were seen from some GRBs. This
allowed the host galaxies to be identified and distances
to be measured.
• The luminosities of these things are HUGE: 1052 ergs/s.
Equivalent to vaporizing a star into pure energy in a
matter of 10 seconds.
• Also highly beamed and relativistic (indicating jets
moving near the speed of light)
• Now we have the satellite Swift that can rotate quickly in
the sky to do detailed studies of GRBs
Long and short GRBs
Further observations show that there are two classes:
• Long gamma-ray bursts (> 2 sec): Found in young,
star forming regions. Some are clearly massive
supernovae (hypernovae?) because spectra are seen.
• Short gamma-ray bursts (< 2 sec): Found in young
and old regions. Thought to be two merging neutron stars
or a neutron star plus a black hole.
In both classes, the “central engine” is the same: a
black hole rapidly accreting from a massive
accretion disk
The difference in times basically reflects the different
reservoirs of mass available to accrete in each case.
Milky Way Galaxy: Our Home!
• A huge, rotating, gravitationally bound system of about 400
billions stars. The Sun takes 250 millions years to orbit, so it
is about 18 orbits old.
• A spiral galaxy: disk with spiral arms in which new stars
tend to form.
Side view
(not to scale)
Nucleus
bulge
1000 ly
Sun
Globular
clusters
Halo
80,000 ly
Are we at center of our Galaxy?
• The gas and dust in the disk of the galaxy absorb and
scatter much of the light. Also called extinction or
obscuration. So we can’t “see” very far at visible
wavelengths.
• Because of this, we seem to be at the center if we just
count stars in different directions.
We are NOT at the center!
• Harlow Shapley (1917) noticed that globular clusters seem
very concentrated in one direction.
• By assuming that globular clusters orbit the center of the
galaxy, he derived the distance from the Sun to the center.
The Milky Way in the Sky
• Band of light stretching across the sky on a dark, moonless
night: “Milky Way” (countless faint stars, gas, dist in plane of
galaxy)
Sun
• Very splotchy distribution.
• Nebulae (clouds of gas and dust):
Emission nebula: Gas is ionized by ultraviolet light
from hot stars. Glows at optical wavelengths when
electrons recombine with ions, or when electrons
excite atoms and ions by colliding with them.
Reflection nebula: Lots of dust and gas reflects light from
adjacent stars.
Dark (absorption) nebula: Blocks light.
Types of Nebulae
Dark nebulae generally
have lots of dense gas
and dust.
Reflection nebula
UV
Hα emission
Diffuse gas, clouds of gas and dust between stars: the
interstellar medium (ISM). The raw material for new stars!
Radio Observations of Our Galaxy
• Because of all the dust and gas in our Galaxy, longer
wavelength observations are key.
• Radio observations are very important for this
reason. But where does radio come from?
• Hydrogen spin-flip transition: emits a 21 cm photon!
• Molecules: water (H2O), ammonia (NH3),carbon
monoxide (CO), molecular hydrogen (H2), and hundreds
of other molecules are seen in the radio
• These molecules weren’t initially expected (gas too
low-density for molecule formation), but we now know
that dust grains help to allow molecules to form.
“Rotation Curve” of the Milky Way
For a large part of the distance out from the center of the
Milky Way Galaxy, the speed of orbiting objects is just about
constant. It is said that the rotation curve is “flat”
Why don’t arms wrap up quickly
Solution: Arms are gas “density waves” of compression that
rotate more slowly than the galaxy disk. Massive stars form there.
Analogy: Cars moving past a slowly moving road block
bottleneck. Road block causes a moving overdensity of cars.
Other Galaxies
• Originally called “spiral nebulae” because of their shape.
• A big controversy in the early 1900s was: Are these
nebulae gas clouds in Milky Way or other “universes”?
• Edwin Hubble - 1924 - observed very faint “Cepheid
variable stars” in some of them.
• Cepheids are evolved supergiant stars that brighten
and fade periodically as their size oscillates. Remember,
these are standard candles.
• If Cepheids appear faint, then they must be way
outside the Milky Way Galaxy.
• From the distance and angular size of spiral nebulae,
Hubble deduced that they are HUGE stellar system,
“island universes”