A105 Stars and Galaxies

This week’s units: 70, 71, 72, 73 News Quiz Today Milky Way homework due Thursday 3 observing events

Today’s APOD

Announcements…• Kirkwood Obs. open

–Weds night 7-9 PM• Rooftop Thursday 8 PM• Transit of Mercury Weds

–2:15 – 5:15–Kirkwood & Sample Gate

Transit of Mercury

• Or view it online at:

www.exploratorium.edu/transit/

Introducing: The Milky Way

Our very own spiral galaxy

Four Galaxies

similar to the

MW

Barredspirals

(seen face-on)

The Andromeda Galaxy

Almost a twin of the Milky Way…

Just bigger

Exploring the Milky Way

Virtual Reality All-Sky Milky Way Panorama

The Milky Way….

Halo

Dwarf Spheroidal Companions

Dark Matter Corona

Bulge Disk

Halo

Where is the Center of the Milky Way?

Measuring the True Size and Shape of the

Milky Way

• The discovery of certain types of variable stars allowed Harlow Shapley to determine the distances to globular star clusters

• Globular clusters concentrate near the center of the galaxy.

Globular Cluster Locations

The Galactic Center

The Sun

The distance to the Galactic Center is about 8 kiloparsecs (8,000 parsecs), or about 26,000 light years

Key Ideas

• The Milky Way is a spiral galaxy• The Galaxy is shaped like a disk• The Sun is located at the inner edge of a spiral

arm about 2/3 of the way out from the center to the edge

• The main components of the Galaxy are the disk, the bulge, and the halo

• We can measure the mass of the Galaxy from the orbits of stars

THE MILKY WAY

Draw two sketches of the Milky Way from different perspectives

Add as many details as you can

Milky Way Factoids

• The Sun orbits the center of the Galaxy– moving in a direction toward Cygnus– speed of about 220 kilometers/second– One orbit takes about 240 MILLION years

• The mass of the Galaxy is about 2x1011 times the mass of the Sun

The Milky Way – Review Vital Stats

• Consists of 100 billion stars.• Stars are distributed in a central bulge, a

huge disk, and a galactic halo surrounding both.

• The diameter of the disk is 30kpc (100,000 light years).

• The thickness of the disk is only 300pc (1000 light years) on average.

• The total detectable mass is 200 billion solar masses.

Our Local Neighborhood• The Sun is located on the outskirts. • Most stars in our neighborhood are cool, low mass

stars• There are no massive (O or B) stars nearby• The interstellar gas in our neighborhood is still quite

hot It is a graveyard of recent massive star deaths.

Galactic Inhabitants

• Stars (of all masses) – disk/bulge

• Star clusters – open clusters (near the disk), globular clusters in the halo

• Clouds – giant molecular clouds, H I clouds, H II regions (disk)

• Nebulae – reflection nebulae, emission nebulae, nova/supernova remnants, planetary nebulae – disk

• The Disk– Gas – ionized hydrogen (H II), atomic hydrogen (H I),

molecular hydrogen (densest and coldest), stellar ejecta– Dust– Stars and failed stars (brown dwarfs)– Stellar remnants – black holes, neutron stars, white

dwarfs

• The Halo– Is virtually gas free which implies little star formation

and chemical enrichment– Consists mostly of old, metal-poor stars (for example,

in the globular clusters)

Stellar “Populations”

Stars formed at a particular “epoch” of the history of the Milky Way share similar properties such as composition and motion.

Structure of the Disk

What Sustains the Spiral Pattern?

• The stars in the galaxy revolve around the galactic center due to gravity

• The galactic disk rotates differentially – stars near the center move faster than those farther away

• This produces a spiral structure but should quickly wind up after a few rotations and disappear

• What then preserves the arms?

One origin of density waves

The Density Wave Theory

• According to this theory, the spiral arms are not material but are rather waves or ripples which cause greater concentrations of stars and interstellar dust and gas.

• Stars move in special, precessing elliptical orbits which gives rise to the spiral pattern.

• The spiral pattern moves as a shock through the galaxy, compressing gas into molecular clouds, promoting star formation at the trailing edges.

The Milky Way

Origin of the Milky Way I

A huge, million-parsec-sized blob of gas begins to contract under gravity. The first stars and star clusters form.

Origin of the Milky Way II

The rotating cloud of gas begins to contract toward its equatorial plane.

Origin of the Milky Way III

Stars and clusters are left behind in the “halo” as the gas cloud flattens.

Origin of the Milky Way IV

IV Stars and clusters formed in the flattened cloud remain in the newly formed “disk”

Origin of the Milky Way V

The disk is now very thin, and the bulge has formed

Throughout the process smaller galaxies are cannibalized as the Milky Way grows

The Milky Way Is Still Growing

• Nearby dwarf galaxy discovered in 1994 in the direction of Sagittarius

• Discovered by radial velocity

• Distance about 88,000 light years

• Merging with the Milky Way

• Orbits the Milky Way• Orbital period about a billion years• “Tidal stream” of stars from Sagittarius circles the Milky

Way• Sagittarius may contain significant dark matter

Sagittarius Tidal Stream

Yet Another New Galaxy!

• Canis Major Dwarf • Nearest galaxy to the Milky Way (yet

discovered…)• 25,000 light years from the Sun• 44,000 light years from the center of the Milky

Way• Discovered with IR light (hidden behind dust in

the MW’s disk)

Tidal Streams from CMa Wrap around the Milky Way

Read Units 70, 71, 72, 73 Milky Way HW Due THURS. 3 observing events available