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HW #2 due on Friday

Color version of Prob 8, clickable links to the Youtube videos available on Carmen

Please turn off all electronic devices

Don’t forget to sign the attendance sheet

How can you determine the distance to a Cepheid star?

a) Measure its period

b) Measure its luminosity

c) Measure its period and brightness

d) Measure its brightness

e) Measure its spectrum

Lecture 13:

The Discovery of Galaxies

Astronomy 1143 – Spring 2014

Key IdeasNature of Nebulae – important scientific question

• Objects inside MW or distant galaxies like MW?• Problems:

• “nebulae” includes several different phenomena

• inaccurate stellar distances

• inaccurate measurements of motion

Accurate stellar distances established…• We are in the Milky Way, a spiral galaxy• Milky Way is one of many galaxies in Universe• Different galaxies – different kind of star orbits

Ancient Observations of GalaxiesIn the southern skies, there are two galaxies – star systems held together by gravity outside of the Milky Way

Not visible from most of the North

Mentioned 964 A.D. by Al Sufi– visible from the Strait of Babd al Mandab

Probing the skies

The return of Halley’s Comet in 1758 made comets very, very popular

All astronomers wanted to discover one, so they used their telescopes to sweep the skies looking for faint, fuzzy objects

If it were a comet, it would move from night to night

If it didn’t move, it was disappointing.

Charles Messier cataloged these objects…

Fuzzy Objects in the Sky:

The Nature of the Nebulae

With telescopes, astronomers found fuzzy things in the sky

Called them “nebulae” -- Latin word for cloud

Were they galaxies like the Milky Way?

Were they clouds of gas inside the Milky Way?

Observations with new and better instruments and new techniques gradually revealed several clues to the nature of these objects.

The Nebular Hypothesis

One popular explanation for the nebulae was that they were forming solar systems – discovery of central stars seemed to confirm

Observations of NebulaeDuring the 19th century, ever larger telescopes

were built.

Some nebulae were seen to have a spiral structure

Spectra of objects – spiral nebulae had spectra similar to stars

Other nebulae, such as planetary nebulae, had emission-line spectra – different phenomena

Observations of Nebulae

Bright outbursts observed in spiral nebulae (such as S Andromedae in 1885)

Are these similar to the novae (rapid brightening of individual stars) seen in the Milky Way?

The spiral nebulae in general have large velocities heading away from us.

There were also observations of rotation.

Are the spiral nebulae like the Milky Way?

Shapley-Curtis DebateShapley: spiral nebulae are not galaxies like MW•Distances large, but not large enough

•Milky Way is very large; spiral nebulae aren’t far enough away

•Events like S Andromedae would have to be much more luminous than Milky Way novae

•Observed rotation cannot be explained if at large distances

Curtis: spiral nebulae are galaxies outside MW•Milky Way is not so big; spiral nebulae can easily be outside

•Appearance of nova says spiral nebulae made of stars

•Large speeds away from us not seen for stars & objects that we know are in the Milky Way

•Rotation measurements are wrong

Rotation and SpeedsYour calculation of how far (in kilometers) the spot in the spiral nebula moves depends on how far you think the object is.

Angular size + distance = physical size.

Hubble Ends the Debate

Edwin Hubble (1923):• Using the new 100-inch telescope on

Mt. Wilson in California. • Found a Cepheid Variable in Andromeda• Shapley’s P-L relationship gave a distance of

300 kpc

By 1925:• Hubble had measured 10 Cepheid variables• The Distance to Andromeda: ~1000 kpc.• Size of the Milky Way: 30 kpc

Hubble’s Cepheid in Andromeda

100-inch Telescope(Mt. Wilson)

Current Understanding

With modern technology and more decades of investigation, we know:

Spiral “nebulae” clearly resolved into stars

There are extremely luminous stellar explosions in galaxies called supernova.

The rotation measurements incorrect

The fact that galaxies are moving away from the Milky Way in general is extremely interesting.

Andromeda (M31)Nearest bright galaxy to the Milky Way:

• Distance ~700 kpc

Many similarities to the Milky Way

• Both are large spiral galaxies

• Both have similar stellar and gas contents

Andromeda gives us an approximate outside view of our own Galaxy.

Galaxies come in many

shapes

Irregular Galaxies

Star motions in different types of galaxies

Spiral Galaxy Elliptical Galaxy

Our Place in the Neighborhood

Obtaining accurate distances for many stars and galaxies led to our understanding of

• The size and shape of the Milky Way and the Sun’s place in it

• The fact that the Milky Way is one of many galaxies in the Universe

• The properties of galaxies outside of our own

• The expansion of the Universe