Context:

I am constantly amazed at how rich every topic in astronomy is. Given the purpose of this class it feels important to stay focused on understanding how science works rather than trying to bury you in masses of information. This class has been structured to build from an exploration of local (as in earth and solar system) to an exploration of the largest scales and timelines. We have now arrived at the one of the larger scale objects in astronomy which are galaxies and their distribution throughout the universe. After trying to get a general sense of galaxies we will focus on one particular feature of galaxies that has changed our thinking about the universe over the last half century.

A good starting place:

People have been looking at the sky through telescopes for a long time. For several hundred years our tools have been good enough to see objects in the sky which are different than stars. These more diffuse objects that sometimes had apparent structure were called nebulae and were cataloged in the Messier Catalogue and in the NGC Catalogue (remember in our previous stellar evolution discussion the HR diagram of M67 and NGC188). There was a great deal of discussion about what these objects were and how they related to the stars we saw around us. In 1920 there was a 'Great Debate' about the nature of these objects and whether they were part of what we understood about the Milky Way or separate. It is important to note that at the time we could not see individual stars in other galaxies. Shortly after this debate our telescopes were able to descern individual stars in the Andromeda Galaxy (M31) which led to our growing awareness that there are enormous structures in the universe and they are remarkably far away.

Here is the Roberts image from 1888 that started people wondering whether the 'nebulae' were something different.

Here is an image of the glass photographic plate that led to the first determinating that this galaxy (M31) was much further away than we thought.

Compare these to our current glorious images of the same object (which is more often referred to as the Andromeda Galaxy) with current telescopes.

We have all grown up with the idea that there are other galaxies out there like the Milky Way where we live and that the Milky Way itself is a spiral galaxy much like the Andromeda Galaxy. Less than 150 years ago there was no such understanding.

HW: Galaxies

If we draw a sphere around our home galaxy (the Milky Way) that has a radius of 500,000 light-yrs (about 5 times the radius of the galaxy) how many satellite galaxies are within that volume according to this list from Wiki of the nearest galaxies? What is the smallest satellite galaxy noted on the list? ... the closest? ... the largest? ... the furthest?

Scale of Galaxies:

Throughout this course one primary focus has been the effort to get a sense of the scale of objects in the solar system. As we step up to galaxies it is not clear that anything we do can quite help us get a handle on the scale of these objects. Remembering that the nearest bright stars to us are about 4 ly away it is hard to grasp that the Milky Way has an estimated diameter of 200 kly. The Andromeda Galaxy has an estimated diameter of 400 kly and it is about 2500 kly (2.5 Mly) away from us.

Returning to our previous scale where the Sun is at Trader Joe's and Neptune is at Camp Sherman we determined that the nearest bright star is about at the moon. On this same scale the Milky Way galaxy has a diameter 2 times that of our whole solar system! Andromeda is twice that big and is 12 times the size of the solar system away from us. It's practically impossible to get a mental grip on. I do hope you have a sense that even though the galaxies are huge it is a little surprising how close the Andromeda Galaxy is. Given our well developed sense of the emptiness of the universe having another large thing only 12 diameters away is startling. It's like finding another earth sized object less than a quarter of the way to the moon.

Let's let that sit for a moment while we address some other questions.

Activity: What are your thoughts about how galaxies form?

Structure and Evolution:

Spiral galaxies like the Milky Way and Andromeda align nicely with our idea that objects in the universe form from the gravitational collapse of large clouds of gas which form stars like our solar system formed planets. The scale is much larger but galaxies are extension of our thinking not a brand new thing (except for the scale).

Deep Field:

The Hubble Space Telescope has been a game changer for our vision of the universe and the structures that exist in it. Among the most stunning are the deep field images like this one. In these images Hubble stares at a particular point in space taking multiple images (this one is the compilation of 800 images over 11 days) and merging them into one that allows us to see deeper (further) into the universe than ever before. This is the deepest image taken with visible light. There are images that look further but they need light from different wavelength bands as we have previously discussed. As you look around this image notice that there are 4 objects (that's all I see) that have crossed flares. These are the only stars in the image. Everything else is a galaxy...

This image covers an portion of the sky which is about 1/10 the size of the moon. Everything is a galaxy and the universe looks like this in every direction. Each of those spots of light is a galaxy like ours with billions of stars and billions of planets. It's just a little mind boggling.

I would point out that we can look further into the universe but for reasons we will talk about in our unit on cosmology the more distant galaxies are invisible because the light they emit is shifted into the infrared.

Activity: What are your thoughts about the place of humans and earth in a universe like this?

Galaxy Types:

With so many galaxies to choose from there are a truly remarkable range of galaxy types. As we usually do in an observational science we start by trying to organize what we see. Then we seek explanations for how this set of possibilities would come to pass. In this collected image of galaxy images from the Spitzer survey older stars appear bluer and younger stars are white and red. Galaxies, we think, are older to the left.

HW: Galaxies

How would you classify each of these three galaxies using the system above? Explain your reasoning.

 

Assignment: HW: Galaxies

Complete and assemble your solutions to all (2) the HW problems listed here. Scan to a pdf and turn in on LMS. Please review HW format expectations for guidance about your homework solutions.

Reading Ahead:

Next time we will continue our explorations of galaxies on the Galaxies II breadcrumb. This will support our discussion about a particularly important feature of galaxies that has changed our thinking about the universe.