Friday, November 17, 2017

Nov 10 - Going out in style: Nebulae at the end of a sun-like star’s life

Our speaker this week was Rudy Montez, an astrophysicist at the Chandra X-ray Center of the Smithsonian Astrophysical Observatory in Cambridge, MA. Rudy studies "planetary nebulae," the ghostly shells sloughed off by dying stars.

Rudy began by showing a gallery of beautiful Hubble Space Telescope photographs of planetary nebulae, explaining how the various colors tell us about the composition of the layers of each nebula. He noted that astronomers always expected planetary nebulae to be spherical, because they're believed to form as the outermost layers of a giant star expand away in all directions from the hot core.

But that's not what we see! Real-life nebulae display amazing, complex structure, including dumbbell-shaped bi-polar lobes. Rudy explained that the leading hypothesis to explain the formation of these lobes is that a donut-shaped ring of dust surrounding the dying star blocks the star's outer layers from expanding in certain directions, forcing them out at the opposite openings of the donut hole. The origins of this ring of dust are not quite clear, but they may be leftover debris from a pair of binary stars orbiting each other at the center of what will eventually be the nebula.

Rudy then discussed his area of particular expertise: X-ray observations of planetary nebulae from the Chandra space telescope, which give us a window onto the hottest, densest central regions. He showed beautiful composite images of Chandra, Hubble, and Spitzer data, which illuminate, respectively, the innermost, middle, and outermost regions of planetary nebulae. Three windows onto these extraordinary objects.

Friday, November 3, 2017

Oct 27 - Cosmic Mergers & Acquisitions: Mergers and Black Holes and the Growth of Galaxies

Our speaker this week was Dr. Jenny Greene, a Professor of Astrophysics at Princeton University.

Much of Jenny’s research concerns some of the most mind-boggling objects in the Universe: the supermassive black holes that exit at the centers of most galaxies. Millions to billions of times more massive than the Sun, these huge black holes are thought to play a crucial role in the development of galaxies.

Jenny began by reviewing what a black hole is: an incredibly dense object. She used the visual of compressing the Sun to the size of Jupiter, then the Earth, then Manhattan; as it gets smaller, the speed you need to escape its gravity gets larger and larger until it exceeds the speed of light. Since nothing moves faster than that, nothing can escape from a black hole.

Even though we can’t see black holes directly, Jenny showed us evidence of the supermassive black hole in the center of our Galaxy. Careful observations of the Galactic center reveal stars moving at phenomenal speeds. Measuring their positions over decades, astronomers can reconstruct their motions and show that they must be orbiting an incredibly dense but invisible object: a black hole.

Astronomers can’t directly see stars moving in other galaxies but their collective motions can be measured very precisely and it is clear that almost all galaxies require a supermassive black hole in their centers to explain these observations. Interestingly, it seems like the black holes’ sizes ‘know about’ the galaxy that they live in: the mass of the black hole can be predicted if the mass of the galaxy is known. How is this possible? Galaxies grow mostly by merging with other galaxies. This process brings in lots of new stars and gas. The gas both forms new stars and feeds the black hole, but as the black hole eats it forms an ‘accretion disk’ of very hot gas that heats up the remaining gas, eventually causing both star formation and its own growth to stop. In this way the size of the galaxy and the size of the black hole can become related.

If this picture is correct we should also see evidence of pairs of black holes, since each galaxy should have brought one along with it. Jenny described one way she and her students have been looking for these binary black holes. When they get close together, they will be orbiting each other at very high speeds — probably thousands of kilometers per second. The light from their accretion disks will be shifted to redder and bluer colors periodically due to the Doppler shift; if we can observe these color-changing black holes it would be good evidence in favor of our cosmological picture. So far the observations haven’t found anything but astronomers are still looking!

-- David Hendel (graduate student)