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Unformatted text preview: Astronomy Picture of the Day Ultraviolet images of the Sun taken over a complete solar cycle, from the SOHO mission Announcements Quiz scores posted. Median score was 7/10 Final exam seating chart is posted- please check the seating chart so you'll know where to go Brown Dwarfs A brown dwarf is an object that forms in the same way that a star forms, but isn't massive enough to become a star The temperature in a brown dwarf's core never gets high enough to start the p-p fusion chain The temperature can be high enough to fuse deuterium into helium, but there's not much deuterium available to burn Brown dwarfs are extremely faint, since they only briefly have fusion reactions occurring in their cores- they just radiate away their internal heat and fade away Classification of low-mass objects Object type Mass range What happens Planets Below about 13 MJup Between about 13 and 75 MJup (or between about 0.013 and 0.075 M ) Above about 0.075 M (or above about 75 MJup) No fusion reactions ever occur Fusion of deuterium to helium can briefly occur Fusion of hydrogen to helium on the main sequence Brown Dwarfs Stars Brown Dwarfs The first brown dwarfs were discovered in the mid-1990s, and now over 1000 have been found They are assigned spectral types L and T (cooler than M stars) Evolution of high-mass stars What happens to stars with initial mass greater than 8 solar masses? Evolution of stars with mass greater than 8 solar masses 1: H burning to He in the core 4: S, Si burning to Fe in the core, O burning to S, Si, Ne burning to O, Mg, C burning to Na, Ne, Mg, He burning to C, H burning to He star becomes a supergiant 2: He burning to C in the core, H burning to He in a shell 3: C burning to O, Ne, Mg in the core, He burning to C in a shell, H burning to He in a shell Core collapse Once the star's core is made of iron, no more energy can be extracted via fusion reactions Even electron degeneracy pressure can't prevent the core from collapsing to a much smaller, denser state Core collapse With no further source of energy, gravity begins to compress the core to a smaller size The core temperature rises to above 10 billion K Black-body radiation becomes so intense in the core that iron nuclei are broken apart Electrons combine with protons to form neutrons p + e n + neutrino Neutrinos carry energy out of the core The core implodes at almost 1/4 of the speed of light The entire process of core collapse happens in about one second! Core collapse The core collapses down to a radius of about 10 km The outer layers of the star are blown off explosively This is called a Type II Supernova explosion 99% of the energy in a Type II supernova comes out in the form of neutrinos! Core-Collapse Supernovae Some massive stars can eject much of their outer layers in a wind, or lose their outer layers to a binary companion, before exploding H He Heavier elements He Heavier elements Heavier elements Type II The star has retained its outer layers. Supernova has spectral lines from H and He Type Ib Outermost layer of star was ejected before the supernova occurred. Supernova has spectral lines from He but not H. Type Ic Even more mass lost. Supernova does not have spectral lines from H or He. 1062 LI ET AL. Hubble close-up view of the supernova location, from an image taken before the supernova occurred Supernova 2005cs in the galaxy M51 Fig. 1.-- Comparison of the Whirlpool Galaxy ( M51) before and after the SN 2005cs explosion. North is up, and east is to the left. The total field of view for each image is 5A96 ; 9A56. Left: Color-composite HST ACS mosaic image. The position of the SN 2005cs progenitor is marked by a black arrow. Right: Same field, as imaged by the MegaCam instrument on the 3.6 m CFHT on 2005 July 2.28. SN 2005cs is marked by a black arrow. [See the electronic edition of the Journal for a color version of the left panel of this figure.] Since the stars that explode as corecollapse supernovae are luminous supergiants, sometimes we have images that show the actual progenitor star before it exploded TABLE 2 HST ACS and NICMOS Observations of SN 2005cs Exposure (s) 960 480 240 255.72 511.71 Data Set UT Date Jul Jul Jul Jul Jul Instrument ACS ACS ACS NICMOS NICMOS Aperture HRC HRC HRC NIC3 NIC3 Filter Prop. ID 10182 10182 10182 10182 10182 J90ZP1010 ................. J90ZP1020 ................. J90ZP1030 ................. N90ZQ1020 ............... N90ZQ1030 ............... 2005 2005 2005 2005 2005 11 11 11 13 13 F220W F250W F330W F110W F160W position measured from the ACS HRC image is transformed to 2.3. Registration of the HST NICMOS Observations the ACS WFC image, it is coincident with our progenitor canTo determine whether the progenitor star is also seen in the didate, with a difference of X 0:12 ACS pixels (0B006) and pre-SN HST NICMOS observations, we attempt to match geometY 0:10 ACS pixels (0B005). In Figure 2 we show a comparFig. 3.--A 2 00 ; 2 00 close-up of the NICMOS images showing SN 2005cs with the pre-SN F435W, F555W, F658N, and F814W images. North is up and east is to the left in ea rically the SN 2005cs environment in the HSTACS ison between the ACS HRC F250W image and the ACS WFC NICMOS images. Out of the five NICMOS passbands in which F814W image after image registration, while in Figure 3 we show The position of the progenitor measured from the F814W image is marked as a circle in each panel. The progenitor is apparently not detected in the F435W, F5 M51 was imaged, we consider only the F110W (J ), F160W the SN site in all four ACS WFC passbands. We now conclude F658N have identified in the object F222M (K ) exposures southwest of the SN with a high degree of certainty that weimages. A bright blue(H ), andimmediately to theto be deep enough to be use- 2005cs progenitor is likely to be a compact star cluster. ful for our purpose. F814W image the progenitor of SN 2005cs (Li et al. 2005b). Measurements show that the star that exploded was a red supergiant with an initial mass around 10 solar masses All of the heavy elements were created inside stars or during supernova explosions, and then expelled into interstellar space Supernovae and the origin of the elements SN Ia produce iron and other heavy metals Core-collapse supernovae produce C, O, Ne, Mg, Si, and other elements, and also some iron and other heavy metals The ejected material is "recycled" into new generations of stars and planets Without SN explosions, we would not have earthlike planets, organic chemistry, or life! Supernovae in the Milky Way A galaxy like the Milky Way typically experiences about 1 or 2 supernova explosions per century The last recorded supernovae in the Milky Way were in 1572 and 1604 More have probably happened since then, but may have been hidden by dust clouds Supernova remnants The Crab Nebula: Remnant of a Type II supernova explosion in 1054 A.D. Supernova remnants The Vela supernova remnant The Cygnus Loop SN 1987A A Type II supernova that occurred in the Large Magellanic Cloud SN 1987A The star that exploded was a blue supergiant It originally was about 20 times the mass of the sun Neutrinos from SN 1987A In 1987, there were 2 neutrino detectors operating, to detect and study solar neutrinos One was the Irvine-Michigan-Brookhaven experiment, in an underground salt mine in Ohio 19 neutrinos were detected from SN 1987A The neutrinos arrived 3 hours before any light from the supernova was seen Neutron Stars A neutron star is the remnant left over after collapse of a massive star's iron core Composition: basically a solid ball of neutrons! It is supported against gravity by neutron degeneracy pressure The maximum possible mass of a neutron star is about 3 solar masses- at higher masses the degeneracy pressure would be overcome by gravity and the neutron star would collapse into something even more dense! Neutron Stars Neutron stars are typically about 1.5 times more massive than the sun, but only about 10 km in radius A cubic centimeter of neutron star matter has a mass of about a billion tons! This is a billion times more dense than white dwarf matter! Neutron stars How can we detect neutron stars observationally? Pulsars: spinning neutron stars emitting radio waves X-ray binaries: neutron stars accreting matter from a binary companion Pulsars First pulsar discovered by Jocelyn Bell, in 1967 She found a radio source that emitted regular "pulses" of radio waves every 1.337 seconds, like clockwork Pulsars A pulsar is a rotating neutron star Its strong magnetic field generates radio emission The beam of radio waves "sweeps" past us as the star rotates The fastest-spinning pulsars are rotating about 1000 times per second! The Crab Pulsar Hubble image: visible wavelengths Chandra image: X-rays X-ray binaries In a binary system, a neutron star can accrete mass spilling off of a companion star neutron star accretion disk Red giant or main sequence companion star Evolution of high-mass stars: recap Pre-main-sequence: luminosity generated by gravitational contraction (for stars with initial masses above 8 M ) Main sequence: Luminosity generated by H fusing to He. Star becomes a supergiant. Further nuclear burning to heavier elements in the core, and in shells around the core Iron core: no further energy can be extracted Initial mass between about 8 and 25 M : core collapse, followed by a Type II supernova, leaving a neutron star remnant Initial mass greater than about 25 M (?) - collapse to a black hole ...
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This note was uploaded on 04/20/2008 for the course PHYS 20A taught by Professor Staff during the Fall '02 term at UC Irvine.

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