Announcements 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 Ideas Nature 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 Galaxies In 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 Nebulae During the 19 th 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 Debate Shapley: 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 Speeds Your 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