The Milky Way Galaxy A galaxy is a collection of stellar and interstellar matter – stars, gas, dust, neutron stars, black holes – held together by gravity.
This is what we see if we look towards the Galactic center from our vantaggpe point on Earth.
We live in the Milky Way Galaxy. Looking at other spiral galaxies gives us an idea of how our galaxy looks.
Andromeda Galaxy – 3 million lyr
3 main parts of a spiral galaxy:
•Galactic disk
•GlGalacti c b blulge
•Galactic halo In the 18th century William Herschel estimated the size and shape of our gal axy b y si mpl y counti ng st ars i n all di recti ons (assumed all stars are the same luminosity). Sun
What didn’t Herschel know about? DUST! (+ all stars are NOT the same luminosity) More dust along the disk causes the distribution of stars to drop-off artificially – objects more than a few kpc away are hidden by all of the dust. Variable Stars -stars whose luminosityyg change with time because they are physically pulsating There are 2 types of Variable Stars:
RR Lyrae Cepheid
•All pulsate in the same way •Each pulsate somewhat •Periods 0.5 to 1 day differently •Periods range from 1 to 100 days • Variable stars are located on the “instability strip” in the HR diagram • The star is internally unstable • TtddiTemperature and radius vary in a regular way causing pulsations
Cepheid variables are mostly hi gh -mass stars evolving across the top of the HR diagram RR Lyraes are lower mass horizontal branch stars There is a relationship between the pulsation period of a variable and its luminosity.
We can see Cepheids in nearby galaxies!
What does this mean?
They can be used to determine distances!!
Luminosity Flux ∝ distance2 Edwin Hubble Discovered Cepheids in the spiral nebula in the constellation Andromeda in the 1920s His notes about a variable star
Hubble then derived the distance to Andromeda, and showed that it was external to our Galaxy ((3~3 million light years away) – another galaxy in its own right!
This had a profound impact on our Note the date: 6 Oct 1923 understanding of our place in the universe, similar to the Copernican revolution. RR Lyraes are too faint to be seen in distant galaxies.
BUT
They can be seen in Globular Clusters - tightly bound groups of stars in our galaxy.
In the early 20th century , Harlow Shapley observed RR Lyraes in GCs and determined 2 things:
•Most GCs are at great distances (1000s of pc) from the Sun •Their 3-d distribution is a roughly spherical volume of space ab30kbout 30 kpc across Globular Clusters in our Galaxy
Shapley realized that the GCs map out the true extent of our galaxy! Galactic Halo
The hub of the galithlaxy is the Galactic Center - about 8 kpc from the Sun The Structure of our Galaxy
Disk is about 300pc thick at the Sun
Globular clusters reside in a nearly spherical “halo.” Globular clusters (and halo stars) are 10 – 15 billion years old (red in color).
Clusters (and stars) in the bulge are similarly old.
The disk contains a mix of stellar ages – from intermediate ages to still forming… •yygoung O and B stars •young open clusters •nebulae from which stars form The disk also contains considerable amounts of gas and dust, while the halo does not… Why is there so little st ar f ormati on i n th e h al o? Why are halo stars and GCs deficient in heavy elements? Components of the Milky Way 1. Disk (approximately Solar metallicity) a. Stars - mostly young stars many in open clusters - all have ages less than 7 billion years b. Gas - HI (neutral Hydrogen) and HII (singly ionized Hydrogen) c. Dust
2. Halo (metal -poor) a. Stars - mostly old stars some located in globular clusters b. Gas - a small amount of HI, no HII c. Dust - little or none
3. Bulge (metal -rich) a. Stars - mostly intermediate to old stars some in globular clusters b. Gas - little or none c. Dust - little or none How are things moving in the galaxy? Doppler shifts reveal orbital motions about the Galactic Center:
The Sun, at 8 kpc radius, orbits at 220 km/s.
With one orbit taking 225 million years. These data suggest a model for the formation of our Galaxy:
Stars formed early keep their random orbits. The rotation causes the dust to flatten in a disk When gas + dust clouds collide they experience “friction,” which tends to organize their orbits.
Many stars form later, in the disk. How did our Galaxy get its spiral arm structure???
Many galaxies look Some look like No galaxies look like this. this. like this.
Galaxies are old enough for many orbits of the stars.
Stars nearer the Galactic center orbit faster, so why doesn’t the spiral structure “wind up?” A leading theory for galactic spiral arms is spilditiral density waves. Density waves are somethinggy that everyone over 16 y ears of age are familiar with…… Galactic Spiral Arm Structure: Spiral Density Waves
The spiral wave pattern is nearly stationary (it moves around as well but not as much), while the gas, dust and stars move through it. An alternative theory is that of Self-propagating Star Formation
The formation of stars drives the waves – shock waves from the later evolution of stars creates denser regions where new stars are created. HdHow do we measure the mass of the Galaxy?
• From Kepler’ ss3 3rd Law (as modified by Newton):
2 3 P (in Earth years) = a (in AU) / Mtotal (in solar mass units)
• Parameters of the Sun’s orbit around the Galactic Center:
radius (= a) = 8 kpc = 1. 65 × 109 AU period (=P) = 225 million years
10 → Mtotal ≈ 9 × 10 Msun ≈ 100 billion Suns! Mass of the Galaxy: Rotation Curve
• In th e case of 2 b odi es orbiti ng each oth er ( e.g. E arth orbiti ng the Sun )
Mtotal is just the Sun + Earth mass ≈ Msun.
• In the case of the Sun orbiting around the Galaxy, what is Mtotal?
• AditNtMAccording to Newton, Mtotal ithSis the Sun ’s mass p lus the mass o fthf the Galaxy interior to the Sun’s orbit.
• The orbital speed is: v = sqrt(G Minterior / radius) • The orbital speeds of the planets orbiting the sun follow
v = sqrt(G MSun / radius)
• Inside the Galaxy, Minterior increases with radius , so velocity may stay constant or even increase with radius.
• Outside the Galaxy, as in the Solar System , Minterior =M= Mtotal and again v = sqrt(G Mtotal / radius). Mass of the Galaxy: Rotation Curve At 40 Kpc, rotation speed 11 Edge of visible Galaxy at 15 kpc - yields 6 × 10 Msun v ∝ sqrt(M / radius) 11 interior rotation speed yields 2 × 10 Msun
“Dark Matter” Problem
Within Galactic bulge - If the mass ended at 15 kpc, we spherical distribution → v ∝ r should find v ∝ sqrt(1/radius). “Dark Matter” Problem – 2/3 of the Galaxy’s mass is invisible!?! – mostly beyond the visible light radius!
And here…
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And ← There is lots of “stuff” out here → And here… here… Dark Matter candidates:
White Dwarf Stars
Very Low Mass Stars – Red Dwarfs (0.2 Msun)
Brown Dwarfs (<0.08 Msun)
Neutron Stars Should be far too few. Black Holes
Exotic sub-atomic particles “Weakly Interacting Massive Particles” – WIMPs The Search for Stellar Dark Matter (brown or white dwarfs): Massive Compact Halo Objects – MACHOs
•The faint foreground object ( bhibrown or white dwarf) bends the light of the background star because of its gravitational field
•The light from the background star is focused or “l ense d” b y this effect and the star appears brighter. What’s at the center of the Milky Way Galaxy?
Visible light images show many stars but dust hides many more.
In IR and radio wavelengths, we see MANY stars (a million times more dense than the solar neighborhood). Sagittarius A • bright radio source at the center of the Galaxy
Sagittarius (Sgr) A* • object at the very center of the Galaxy • million times more luminous than the Sun (IR , radio , X- ray, and gamma ray Massive Black Hole!! source) Black Hole at Sgr A*
• entire region less than 10 AU across • Stars and gas near Sgr A* are moving fast! • Mass of the black hole - 2 to 3 million solar masses! • Radiation arises from the accretion disk Black Hole here
100 kpc 1 parsec (over 200000 AU)