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Galaxy Classification. The Hubble Tuning-Fork Diagram. This is the traditional scheme for classifying galaxies:. Elliptical Galaxies.
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The Hubble Tuning-Fork Diagram This is the traditional scheme for classifying galaxies:
Elliptical Galaxies Elliptical galaxies are spherical in shape. They contain very little gas and dust, no star formation, and only older stars. They resemble globular clusters, except they are much larger.
Sa and SBa Galaxies Sa and SBa galaxies have a large spherical distributions of stars (called bulges) and a smooth disk with only a trace of spiral arms. Sa (no bar)
Sa and SBa Galaxies Sa and SBa galaxies have a large spherical distributions of stars (called bulges) and a smooth disk with only a trace of spiral arms. SBa (bar)
Sb and SBb Galaxies In Sb and SBb galaxies, the bulge and disk components are comparable in dominance. Spiral structure is visible in the disk. Sb
Sb and SBb Galaxies In Sb and SBb galaxies, the bulge and disk components are comparable in dominance. Spiral structure is visible in the disk. SBb
Sc and SBc Galaxies Sc Sc and SBc galaxies have little or no bulge and a large disk.
Sc and SBc Galaxies Sc and SBc galaxies have little or no bulge and a large disk. SBc
Irregular Galaxies Irregular galaxies are do not have an ordered structure, and instead have random, messy shapes. They have lots of dust, gas, and young stars.
Active Galaxies The light from most galaxies is just the sum of light from all of the stars within it, so like starlight, a galaxy’s light is brightest at optical wavelengths and fainter at shorter and longer wavelengths.
Active Galaxies But a small fraction of galaxies are different; they are much brighter and produce more long- and short- wavelength emission. They are called active galaxies.
The first active galaxies: Quasars In 1962, a radio survey of the entire sky revealed a few peculiar “stars” that were very bright at radio wavelengths (normal stars do not produce much radio emission). They were named quasi-stellar radio sources (quasars, or QSOs):
The first active galaxies: Quasars In 1962, a radio survey of the entire sky revealed a few peculiar “stars” that were very bright at radio wavelengths (normal stars do not produce much radio emission). They were named quasi-stellar radio sources (quasars, or QSOs): …Quasars look like single points of light Instead of this…
Quasars: Fast, distant, and very bright Quasars have enormous redshifts, indicating that they are moving away from us at more than 90% of the speed of light. Stars in the Milky Way cannot move that fast. The only way to achieve such a high speed is if they are incredibly far away. They are therefore incredibly bright – as bright as 1000 supernovae, and much brighter than galaxies like the Milky Way.
Quasars: brightness varies rapidly The brightness of quasars varies rapidly, on timescales of just days and months.
Size of a Quasar’s Energy Source • Imagine that there is some mechanism near the center of the quasar that controls the object’s brightness. It says “get bright”. • That command goes forth no faster than the speed of light. • Within a few months, the object gets bright. • Since no signal can go faster than the speed of light, the object must be no bigger than a few light-months across. Since many quasars vary in brightness we have a crude way to estimate their size.
A Quasar’s Energy Source What can outshine ~1000 supernovae for millions of years, and be just slightly larger than our Solar System? Theoretically, not much – only a very, very big black hole. • Start with a black hole with a mass of 10,000,000,000 Mʘ • Have a star come close enough to be tidally disrupted • Have the material form into an accretion disk. Energy is released via the friction in the disk. If you accrete ~ 1 Mʘ per year, the friction you get will produce the luminosity of a quasar.
Feeding the Monster If a star comes too close, the enormous gravity of the black hole will cause tides on the star and rip it apart. Some of that material will be trapped in orbit about the hole.
Because of the high speed of the gas, there is a lot of friction in the disk. This is why quasars are very bright. Explaining a Quasar’s Properties • The temperature of the disk depends on the speed of the gas. Near the event horizon, the friction produces x-rays. At larger radii, where the gas revolves more slowly, optical and infrared light is made. This is why quasars are bright at all wavelengths.
Black Holes and Jets As matter accretes onto the black hole, particles can get ejected out the poles of the system at 99.999% of the speed of light. How this occurs is almost a complete mystery. But it’s often observed.
Black Holes and Jets As matter accretes onto the black hole, particles can get ejected out the poles of the system at 99.999% of the speed of light. How this occurs is almost a complete mystery. But it’s often observed.
Where are the Quasars Today? The nearest quasar is 25% of the way across the universe; most belong to an era when the universe was only 15% of its present age. If supermassive black holes existed then, where they now? In the centers of galaxies!
How to Make an Active Galaxy When a supermassive black hole is not accreting, it is invisible, and the galaxy is “normal”. But when it is accreting, it is thousands of times brighter than its surrounding galaxy, and the galaxy is “active”.
The Milky Way’s Sleeping Monster There’s even a 2,000,000 Mʘ black hole at the center of the Milky Way. We can measure its mass by the motions of stars which pass close to it.
