milky way properties Flashcards
Everything we know about the Galaxy’s
innermost region comes from
Radio
* Infrared
* X-ray
observations
The nucleus of the Galaxy is hidden from
us at visible wavelengths
Dust in the disk makes objects a trillion
times fainter
The radio center of the Galaxy
Observed at a wavelength of 1-
meter (from VLA), the center
of the Galaxy is a complex
and crowded region
Filled with supernova
remnants, ionization
nebulae, and filaments of
gas.
The Galactic center lies in the
brightest part, Sagittarius-A
(Sgr-A)
Producing radio emission
- hot ionized gas and
supernova shock waves
both produce radio
emission - electrons spiraling along
magnetic fields produce
radio “synchrotron”
emission
X-rays penetrate all but the densest dust clouds. At the center, we find
thousands of X-ray binaries (white dwarfs, neutron stars, black holes)
* a haze of extremely hot gas, heated by winds from hot, young stars
Sagittarius-A
Sgr-A has three distinct parts:
* a young supernova remnant on
the east side
* spiral structure on the west
side
* a point source, Sgr-A*
The center in the infrared
Stars are not visible at radio or X-ray
wavelengths
IR observations reveal 10 million
stars within one parsec of the
center
A cluster of thousands of stars is
centered on Sgr-A
mostly old main sequence stars
* ~100 hot, young (O&B) stars
How do we learn about what is going on in the center of
our own galaxy?
We cannot see the galactic center with visible or UV light, but
radio and X rays from the center can be detected
The formation of supermassive
black holes is an open questio
stellar mass black holes that
merged and grew over time
* initial collapse of massive
clouds into a black hole when
the galaxy formed
High speed stars near our Galaxy’s center are explained
by
a supermassive black hole
Andromeda Galaxy (M31)
Nearest bright galaxy to the Milky Way
* 2.5 million light years away.
Many similarities:
* both are large spirals
* Similar gas and star content.
Andromeda give us an approximate
view of our own galaxy.
two populations
In the thin disk
stars tend to be younger, rich in
heavy metals, and travel circular
orbits in a plane at high speed
populations in halo
Disordered, elliptical orbits at all
inclinations
* stars are older, poor in metals,
and travel orbits inclined to the
plane, some very elliptical
Compared with stars in the disk, orbits
of stars in the halo
are elliptical, with random orientation
Population I
Location: Disk and Open Clusters
Age: Mix of young and old stars
(0-10Gyr)
Composition: Metal rich (roughly
solar)
70% Hydrogen
28% Helium
~2% “metals”
Environment: Often gas rich,
especially for the young stars
population II
Location: Halo, Globular
Clusters, DIsk
Ages: Oldest stars, 11-13 Gyr
Composition: Metal Poor (0.1-
1% solar)
75% Hydrogen
24.99% Helium
~0.01% metals
Environment: generally gas poor,
no star formation
Compared to a Population II star of the same spectral
type, a Population I star is
younger
Clues to Galaxy Formation?
Chemical Evolution only affects populations
* Fusion occurs in the deep interiors of stars.
* A star’s surface composition remains
effectively unchanged over its lifetime.
* Once a star forms, its chemical composition
is mostly fixed for life.
Metal content gives us a clue to the
formation history of populations of stars
first step in galaxy forming
A spherical, protogalactic
cloud of H & He began to
collapse under gravity 12-
13 Gyr ago
second step in galaxy forming
First generation of stars was metal-poor
halo & globular cluster stars (Pop II)
* Massive Pop II stars went supernova
and enriched the gas with metals.
* Only low-mass Pop II stars are still
around today
third step in galaxy forming
Star formation stopped very early-on
in the halo as gas formed a disk.
* First generation of old Pop I stars
started with more metals in the
rotating disk.
* Massive old Pop I stars went
supernova, enriching the disk gas
further
Star formation stopped first in the
halo
step four in galaxy forming. still forming today
Subsequent generations (young
Pop I) have even more metals
(e.g., the Sun).
* Star formation is still going on in
the disk
