Reminders! Website: http://starsarestellar.blogspot.com/ Lectures 1-15 are available for download as study aids. Reading: You should have Chapters 1-14 read. Read Chapters 15-17 by the end of the week. Homework: Homework #4 is due tomorrow, June 24th. Observing Projects: Both due Monday. They will NOT be accepted late!!!
Our Home, the Milky Way Today s Lecture: GRB Review (Chapter 14, pages 337-339) Two types of gamma-ray bursts Universal central engine The Milk Way (Chapter 15, pages 344-365) The main parts of our Galaxy Nebulae What the spiral arms?
GRBs: Fantastic explosions! In 1991 new telescopes showed GRBs are isotropic on the sky, proving they are (usually) NOT from our galaxy Afterglows (optical) were seen from some GRBs. This allowed the host galaxies to be identified and distances to be measured. The luminosities of these things are HUGE: 10 52 ergs/s. Equivalent to vaporizing a star into pure energy in a matter of 10 seconds. Also highly beamed and relativistic (indicating jets moving near the speed of light) Now we have the satellite Swift that can rotate quickly in the sky to do detailed studies of GRBs
Long and short GRBs Further observations show that there are two classes: Long gamma-ray bursts (> 2 sec): Found in young, star forming regions. Some are clearly massive supernovae (hypernovae?) because spectra are seen. Short gamma-ray bursts (< 2 sec): Found in young and old regions. Thought to be two merging neutron stars or a neutron star plus a black hole. In both classes, the central engine is the same: a black hole rapidly accreting from a massive accretion disk The difference in times basically reflects the different reservoirs of mass available to accrete in each case.
Milky Way Galaxy: Our Home! A huge, rotating, gravitationally bound system of about 400 billions stars. The Sun takes 250 millions years to orbit, so it is about 18 orbits old. A spiral galaxy: disk with spiral arms in which new stars tend to form. Side view (not to scale) 1000 ly Sun Nucleus bulge Globular clusters Halo 80,000 ly
Are we at center of our Galaxy? The gas and dust in the disk of the galaxy absorb and scatter much of the light. Also called extinction or obscuration. So we can t see very far at visible wavelengths. Because of this, we seem to be at the center if we just count stars in different directions.
We are NOT at the center! Harlow Shapley (1917) noticed that globular clusters seem very concentrated in one direction. By assuming that globular clusters orbit the center of the galaxy, he derived the distance from the Sun to the center.
The Milky Way in the Sky Band of light stretching across the sky on a dark, moonless night: Milky Way (countless faint stars, gas, dist in plane of galaxy) Very splotchy distribution. Nebulae (clouds of gas and dust): Emission nebula: Gas is ionized by ultraviolet light from hot stars. Glows at optical wavelengths when electrons recombine with ions, or when electrons excite atoms and ions by colliding with them. Reflection nebula: Lots of dust and gas reflects light from adjacent stars. Dark (absorption) nebula: Blocks light. Sun
Types of Nebulae Dark nebulae generally have lots of dense gas and dust. Reflection nebula UV Hα emission Diffuse gas, clouds of gas and dust between stars: the interstellar medium (ISM). The raw material for new stars!
Radio Observations of Our Galaxy Because of all the dust and gas in our Galaxy, longer wavelength observations are key. Radio observations are very important for this reason. But where does radio come from? Hydrogen spin-flip transition: emits a 21 cm photon! Molecules: water (H 2 O), ammonia (NH 3 ),carbon monoxide (CO), molecular hydrogen (H 2 ), and hundreds of other molecules are seen in the radio These molecules weren t initially expected (gas too low-density for molecule formation), but we now know that dust grains help to allow molecules to form.
Rotation Curve of the Milky Way For a large part of the distance out from the center of the Milky Way Galaxy, the speed of orbiting objects is just about constant. It is said that the rotation curve is flat
Why don t arms wrap up quickly Solution: Arms are gas density waves of compression that rotate more slowly than the galaxy disk. Massive stars form there. Analogy: Cars moving past a slowly moving road block bottleneck. Road block causes a moving overdensity of cars.
Other Galaxies Originally called spiral nebulae because of their shape. A big controversy in the early 1900s was: Are these nebulae gas clouds in Milky Way or other universes? Edwin Hubble - 1924 - observed very faint Cepheid variable stars in some of them. Cepheids are evolved supergiant stars that brighten and fade periodically as their size oscillates. Remember, these are standard candles. If Cepheids appear faint, then they must be way outside the Milky Way Galaxy. From the distance and angular size of spiral nebulae, Hubble deduced that they are HUGE stellar system, island universes