Galaxies I

AST 112

The Universe

Is the Universe contained entirely within theMilky Way?

Milky Way Inhabitants

• Open clusters are groups of newly formed stars

• We can measure the distance to them using Main Sequence Fitting

Milky Way Inhabitants

• Globular clusters’ origins aren’t exactly known

• We measure the distance to them using variable stars

Milky Way Inhabitants

• Nebulae glow because of either:– Embedded stars or– Reflected starlight

• Easy to identify the chemistry

• No well-defined shape

Milky Way Inhabitants

• Occasionally a star would brighten significantly and then fade

– This was referred to as a nova

– Please don’t mix this up with what we’ve covered!

Milky Way Inhabitants

These are objects that were fairly well understoodand whose distance could at least be estimated, if not

accurately measured.

There was one type of object that baffled astronomers:“Spiral Nebulae”

Spiral Nebula

Spiral Nebulae

• Spiral nebulae appear “out of place” in our night sky– They have structure– Could not get distance

estimates

The Shapley-Curtis Debate

• Harlow Shapely:– The Universe IS contained within the Milky Way

• Heber Curtis:– The Universe IS NOT contained within the Milky

Way

The Shapley-Curtis Debate

• The Shapley-Curtis Debate (aka The Great Debate) : “Spiral Nebulae” – Contained in the Milky Way?– Or are their own “Island Universes”?

• The (eventual) outcome would determine our understanding of the size and structure of the universe!

The Shapley-Curtis Debate

• Shapley:

– Spiral Nebulae part of the Milky Way

– Consider sizes:• Shapley overestimated size of Milky Way • Claimed that if Andromeda were not in the Milky Way,

would have to be 100,000,000 LY away to be as large as it is

The Shapley-Curtis Debate

• Ancient Greek astronomers:– 4 LY to the nearest star? Yea right!

• Shapley:– 100,000,000 LY to a “spiral nebula”? Yea right!

The Shapley-Curtis Debate

• Adriaan Van Maanen claimed to observe Messier 101 rotating– Measured degrees per year

• If these objects are truly millions of LY away, Van Maanen’s calculations suggest the galaxies’ edges would break the speed of light

The Shapley-Curtis Debate

• A “nova” occurred in Andromeda that seemed to outshine the entire “spiral nebula”

• How could a “nova” outshine an entire “island universe”?

The Shapley-Curtis Debate

• Curtis:– “Spiral nebulae” are island universes– Cities of stars just like the Milky Way

Andromeda and the Milky Way

What is the most obvioussimilarity that you see betweenAndromeda and the Milky Way?

The Shapley-Curtis Debate

• Curtis argued:– Dust lanes in Andromeda just like in the band of the

Milky Way• Explains Zone of Avoidance

• Vesto Slipher saw large Doppler shifts in other galaxies– Can’t remain gravitationally bound– Transverse velocity should be similar to radial

velocity• Should show proper motion (and it doesn’t)

The Shapley-Curtis Debate

So who is winning at this point?

The Shapley-Curtis Debate

• Curtis admitted:

– If Van Maanen’s observation of M101’s rotation is accurate, I’m wrong

The Shapley-Curtis Debate

• Curtis saw records of 10 “novae” in Andromeda– On average, 1/10000 as bright as here

• Many more than observed in the Milky Way

• So why so many novae concentrated in one small part of the sky?

The Shapley-Curtis Debate

Van Maanen showed his own observation of M101 rotating was incorrect!

The Shapley-Curtis Debate

Now who’s winning?

The Shapley-Curtis Debate

• Debate was a draw but highlighted issues

• Three years later, Edwin Hubble used the 100 inch telescope at Mount Wilson– Saw individual stars– Found Cepheid variable stars in Andromeda– 900,000 LY away (still off)

Cepheid Variable in Andromeda

Scopes are better now!

It is easy to see elementsthat we recognize in our galaxy in images of othergalaxies.

Zone of Avoidance

• Sombrero Galaxy (M104) has dust lanes

• We have a “zone of avoidance” where we cannot see globular clusters

• Easy to look at M104 and assume they also have a “zone of avoidance”

Issues To Reconcile

• We can measure the rotation of galaxies. – We use Doppler shift– We cannot actually see the galaxy rotating, as Van

Maanen claimed to

• The “novae” in Andromeda were of course supernovae– And they do outshine entire galaxies for a short

time!

Issues To Reconcile

“Spiral Nebulae” are galaxies!

They are not nebulae!

We will not use that term anymore!

Cepheid Variable Stars

• Cepheid Variables used for distances to galaxies up to 100,000,000 LY away

• They are bright

• Pulsation period related to luminosity

Type 1a Supernovae

• White dwarf supernovae– Reach 1.4 MSun

Chandrasekhar Limit– “It’s always the same

star” that explodes– Extremely consistent

luminosity

• Bright (10 billion LSun at peak)

Type 1a Supernovae

Tully-Fisher Relation

• Relation between luminosity of spiral galaxies and their rotation speed

• Measure Doppler shifts of each side– Tully-Fisher relation then

gives luminosity