Let’s review last week: The Distance Ladder: compu9ng distance to anything in the Universe, a Universe filled with galaxies, each galaxy filled with stars Our important rule: if we know how Bright something is when its at a know distance,, and we see a similar oBject that is much fainter (thus farther) we can compute the distance of the fainter oBject We call these oBjects “standard candles” Today we will look at a series of related “standard candles” that can Be linked to find distances to the farthest galaxies Review: how do we measure distance to nearBy stars in our galaxy? The Pleiades (SuBaru in Japanese, the Homeless Women in an O’odham story), a cluster of stars visiBle to the naked eye The HR diagram for the Pleiades: we can measure the distance to these stars By parallax, and compute how Bright they would Be compared to the sun, above. Consider a more distant open cluster – too far for parallax! We can measure the apparent Brightness of Chi Persei, the each star: since the stars are all in a cluster, at double cluster the same distance, we can plot this versus the color of each star. When we compare the HR diagrams, we can compute how much fainter Chi Persei is compared to the Pleiades – thus how much farther it is. This is possiBle due to the inverse square law for light: an oBject twice as far away will Be only one fourth as Bright Pleiades Chi Persei For those taking algebra: our “black box” calculator for three quan88es m –M = 5 log (distance) – 5 Distance ladder: 1. Parallax of nearBy stars to find their distance 2. HR diagram of clusters, and comparison with stars we know allows us to find distances (using a property of light known as the inverse square law). Now, we can get preWy good idea of distance to any star who colors and spectrum are known The solar system is surrounded By the Band of the Milky Way We see the nearBy stars in all direc9ons, But the more distant stars merge into the Band we call the Milky Way Suppose we ask what it would look like if we could fly far above and view it. How can we figure out what we would see? We can make a map since we know the distance and direc9on of a star What do we find when we measure distances to lots of open clusters? We already found that they are all within the plane of the Milky Way. But since we are inside the Milky Way, we will measure around in a circle – like azimuth around our local horizon. Here is what we find What do you suppose this is showing us? Hint: The direc9on to 0 degrees is toward the constellaon SagiWarius So we can now draw a “cartoon” of our Milky Way galaxy showing spiral arms RememBer, this is only a cartoon: we have only sent spacecras to the outer edge of our solar system Young stars in open clusters define mulple spiral arms And there is a very massive Black hole in center (Gautham’s lecture) Now, let’s look at gloBular clusters: (where did we find them in the sky?) M2, Izzy & Angel, 0.9m image ( Black and white image – why?) From the HR diagram, what color do you think many of the stars are? They cluster in the direc9on of the constellaon SagiWarius… And if we compute distances to the gloBular clusters (using Bright stars) we find they form a halo around galac9c center But just relying on what we know about the Brightest stars isn’t a great way to es9mate distance – we need another tool! One more method of finding distance: variable stars The Brightness of some stars changes, geng Brighter and fainter, regularly like clockwork, with a period of a few to hundreds of days. Among them, the Cepheids are named aer a Bright variable in the constellaon Cepheus This en9re star is pulsang as it passes through an unstable phase of its lifeme And what makes them a standard candle is that the period of pulsaon is related to the star’s intrinsic Brightness (and if we can determine the intrinsic Brightness, what else can we find?) Bright Cepheids can Be used to get distance to some nearBy galaxies! M33, at 0.9m , Dana, Eli, Francina …from a research paper, measuring Cepheids in M33 Distance ladder: 1. Parallax of nearBy stars to find their distance 2. HR diagram of clusters, and comparison with stars we know allows us to find distances (using a property of light known as the inverse square law). Now, we can get preWy good idea of distance to any star who colors and spectrum are known 3. Variable stars known as Cepheids: period of pulsaon is related to their intrinsic Brightness – can Be used to get distances to other galaxies And now we reach the final step on our distance ladder: supernova! Supernova in M 101, the Pinwheel galaxy (…the distance to M 101 is alreadyknown from Cepheid variable stars) Where are supernova in the life of a star? If star was < 8 x sun In par9cular, supernova in Binary systems always reach about the same intrinsic Brightness: so when we see one, we can compute its distance Ar9st concep9on of a supernova in a Binary star system All supernova in Binary system (astronomers call them type Ia supernova) reach the same maximum Brightness: this makes them a standard candle When we oBserve one, and measure its apparent Brightness, we can then compute its distance. Let’s try an exercise and see what sort of distances we find We start with this galaxy, NGC 4414, whose distance is known from Cepheid variables to Be 60 million ly away In 1974, a supernova was oBserved in it Our table, completed: can you no9ce any paerns? NGC 4414, our first galaxy, whose distance is known from Cepheid variables to Be 60 million ly ( image is 2.9 arc minutes long) sn 1993ac in galaxy LEDA 17787 (0.8 arc min long) SN 1994S in NGC 4495… (1.4 arc minutes long) Suppose we sort the And then make a graph… table By the distance: let’s make a graph of distance versus redshi]. What does this graph suggest to you? Distance ladder, complete: 1. Parallax of nearBy stars to find their distance 2. HR diagram of clusters, and comparison with stars we know allows us to find distances (using a property of light known as the inverse square law). Now, we can get preWy good idea of distance to any star who colors and spectrum are known 3. Variable stars known as Cepheids: period of pulsaon is related to their intrinsic Brightness – can Be used to get distances to other galaxies 4. Supernova at maximum Brightness are all the same (at least the Binary type Ia) so we can use them to get distances to far galaxies – and we find something very interesng! How many galaxies are there in the oBservable universe? HuBBle telescope: an orBi9ng 2 meter telescope, launched in 1990. In 2006 it stared at a Blank piece of sky near the Big Dipper for 10 days to create this image. note: The area of the sky that the camera can image is less than the angular size of one of the dark craters on the moon: about 3.5 arc minutes on a side Let’s do an exercise: How many galaxies are there in the observable universe? The HuBBle Space Telescope images an area about xxx that of the field we imaged with the 0.9m telescope at KiW Peak. In2006, the HST exposed its camera to a very Blank part of the sky – Blank, Because there were almost no stars there. But there were galaxies. we can expect that the numBer of galaxies in this image will Be about the same as in any other direc9on. And since from geometry we know the area of the image, compared to the total area of the sky, we can count the galaxies int his image and mul9ply to find the total numBer of galaxies int eh universe we observe. Each person or team has an image of the HuBBle Deep Field. Begin my making an es9mate of the total numBer of galaxies in this image.. Enter it here: Es9mate of numBer of galaxies in this image: ____________________ Review of this class: Astronomers use different methods to get distance to stars, clusters and finally galaxies – each method Builds on the last • Parallax • Cluster fing, and HR diagram – knowledge of star’s intrinsic Brightness • Variable stars called Cepeids -period of variaon isrelated to intrinsic Brightness • Supernova – all type Ia reach the same Brightness From this, we find: • Our Milky Way galaxy, containing all the stars we see without a telescope, is only one of Billions of galaxies in the universe. • The most distant galaxies we see are so far that light has Been traveling for millions, even Billions of years to reach earth. .