Astronomy 115 – Section 4 Week 13

Astronomy 115 – Section 4Week 13

Adam Fries • SF State • [email protected]

Important Notes

• Start Galaxies, Ch. 11• HW# 4 is posted on the webpage, due at the

Final.• Final is in 2 weeks: May 19th same

time/place• Extra credit is due a week before the final.

Recall:

• Massive stars (Mstar > 8 M�) core fusionstops at iron.• Once the iron core grows to 2 M�, Type II

Supernova (neutron star)• If the neutron star exceeds 3 M�, you get a

black hole.

• Event Horizon is a sphere devoid of light –no reflection or emission• If light crosses the Event Horizon, it cannot

escape the black hole – point of no return• The radius of the sphere is expressed as the

Schwarzchild radius:

RSch =2Gc2 M

Detecting Black Holes

• need to look at binary systems (companionstar orbiting a black hole)• black hole strips off the atmosphere of its

companion• the gas and dust fall into the black hole, but

bottle-necks – heats up and emits X-rays

Ch. 11 Galaxies

• Determine a galaxy’s type by its appearance• how spiral arms form• evidence for dark matter• evidence for supermassive black holes

Difficulty in identifying shapes. . .

There are 3 main galaxy types, and galaxiescome in a wide range of sizes

• Elliptical• Spiral• Everything else – Irregular

Hubble’s Tuning Fork – not an evolutionarytrack

Stellar Motions Give Galaxies Their Shapes

Elliptical Galaxy (Type E), Messier 87 – Virgo

• In ellipticals (Eggs). . .• . . . stars are almost randomly orbiting the

center from a variety of angles• some stars are falling in while others are

climbing out• stars are moving in all possible directions• gas poor, stars have stopped forming• most abundant type of galaxy

Spiral Galaxy (Type S), Messier 81 – UrsaMajor

• In spirals. . .• . . . stars mostly move together as a flat,

rotating disk spiral arms• there is a central bulge, where stars move

more randomly, like in ellipticals• contain large amounts of dust and cold,

dense molecular clouds, stars are stillforming

Barred Spiral Galaxy (Type SB), NGC 1300 –Eridanus

Irregular Galaxy, NGC 55 – Sculptor

• In irregulars. . .• . . . stars move in assymetric orbits• shape is not well-defined• result of galaxy collision• make up nearly 25% of all galaxies

How arms form in spiral galaxies

• Gas (H and He and other metals),• dust (small particles of C and silicon),• hot, young (blue) stars. . .• . . . are concentrated in the arms• (Old, red stars are concentrated in the

central bulge)

• The disk rotates! this naturally produces thearm structure• But to maintain the arms, there needs to be

a sustained gravitational disturbance• Most bulges are elongated (Eggs), this is

enough to sustain the structure• Different shaped bulges generate different

shaped arms

So where are we in the Milky Way (our galaxy)?

By measuring the local density of stars, WilliamHerschel believed that the Sun was at thecenter (1780s). But this was wrong.

Story begins with Henrietta Swann Leavitt

• in 1912, one of the Harvard Computers,Henrietta Leavitt, completed a study on aspecial type of star in a neighboring dwarfgalaxy (SMC)• these stars ‘pulsated’ with a very

predictable period• . . . brightening and then dimming• she deduced a period-luminosity relationship

to find distances with these stars

Small Magellanic Cloud (SMC)

Omega Centauri

Globular Clusters

• Some of the oldest objects in the Universe• Contain 100,000s - 1,000,000s of stars• Some GC are visible to the naked eye

• By 1920, Harlow Shapley used the P-Lrelationship of Cepheid Variable Stars tomap the distribution of 93 globular clusters.• Found that the GC are located in a spherical

distribution not centered on the Earth!• He suggested that the GC orbit the center of

the Milky Way and we are not there.

To find the center of the Milky Way, look for theTeapot!

• In 1970s, it was hypothesized that asupermassive blackhole lived in the center ofthe MW• From 1995 to 2012, Keck/UCLA Galactic

Center Group mapped the motion of starsabout the center. . .

• These are orbits, and orbits follow Kepler’sLaws!• Gross version of Kepler’s 3rd law:

P2 ' 4π2

GMA3

• By measuring the period, P, and semimajoraxis, A, the mass can be found!• SMBH is estimated to be about 3–4 million

solar masses!

Measuring the Mass of a Galaxy

Any Ideas?

We could count up all the light we see from thestars?

But this method leaves out all of the really faintstars, black holes, brown dwarfs (failed stars)

We can use the same trick of estimating theSMBH mass!

Measure the motion (period) of the stars on theouter edge of the Milky Way.

The orbits of these stars should follow Kepler’sLaws, thus we can estimate all the mass interiorto these orbits.

Because the MW is not spherical, astronomersneed to know how the mass is distributedinterior to these outer stars.

It was hypothesized that mass and light weredistributed in the same way throughout thegalaxy.

Wherever there was light, there must be mass.

In the 1970s, Vera Rubin set out to measure theorbital velocities of stars.

What’s going on? Any ideas?

The idea that ’mass and light are distributed inthe same way’ must be wrong!

Astronomers hypothesized that there must beanother component of undetected matter whichis not stars, gas, or dust– this is called DarkMatter

Astronomers believe that the Milky Way’s darkmatter halo mass is ∼ 1012 M�!

So what is dark matter?

So what is dark matter?

Maybe it’s all the stuff we forgot to count like BH,Brown Dwarfs, Neutron stars, White Dwarfs(MACHOS – massive compact halo objects)?

Turns out not to be a good candidate. Notenough lensing events to account for the mass.

How about WIMPS – weakly interacting massiveparticles?

This turns out to be the leading candidate so far.Heavy elementary particles that don’t interactwith normal matter.

Experiments in the LHC are under way to try todetect this exotic matter.

Adjust Newton’s Law of Gravitation at largescales (MOND)?