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Goal: To understand the most energetic stars in the universe, quasars. Objectives: To understand Quasars. Quasars. Quasars are QUASi-stellAR radio sources They were first observed in the 1950s. They looked like stars on an image.
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Goal: To understand the most energetic stars in the universe, quasars Objectives: To understand Quasars
Quasars • Quasars are QUASi-stellAR radio sources • They were first observed in the 1950s. • They looked like stars on an image. • You can tell stars from galaxies because stars appear as a point (and have diffraction patterns if bright) but galaxies are more spread out. • However, when they looked at the spectrum, they found something very strange!
No known elements! • The emission lines matched no known element! • Why would this be? • A) Quasars are made out of materials unknown in the 1950s. • B) They made a mistake with the observations. • C) The lines were Doppler shifted by a factor of a few. • D) They forgot to take relativity into account.
They are far away! • Once it was realized what the emission line difference was they quickly realize that Quasars are very far away! • In fact some are on the other side of the observable universe! • A red flag should go up now – if they are that far away and we can see them – how bright are they? • You have a star which is brighter than any galaxy – how is that possible?
Variable • Furthermore Quasars change in brightness! • They can do so very quickly (in hours). • The shortest a time span an object can fluctuate its brightness is related to its size. • The time is the light speed time across the object. • So, these are extremely bright objects located in a small area (size of a solar system). • How is this possible?
Debate • In the 1970s there was a lot of debate over this puzzling matter. • Some suggested antimatter (the only way they could think of to convert that much energy). • Some suggested that the redshift wasn’t due to distance but due to light coming out of a huge gravitational well. • However the gas emitting the light was hot and diffuse (known by the emission lines) – so could not be.
The solution • The light doesn’t even come from the “star”. • The light is coming from an accretion disk. • As gas and dust orbits friction causes the gas and dust to heat up, emit light, and fall slowly down towards the object in the center. • If the object in the center is massive you can liberate 10% of the mass energy of the dust and gas as energy (as opposed to 0.7% from fusion).
Not the full story • Still, how do we get a region of space the size of a solar system to radiate more light than a galaxy? • Also, this light is radiated in all type of light from gamma rays to radio! • How is this possible?
Black Holes! • Quasars are super massive black holes! • Quasars are billions of times the mass of our sun. • Oddly enough their density is about the same as our sun. • The amount of light they emit is dependant on the rate they accrete matter.
More bizarre news • Most quasars are > 5 billion light years away. • Very few are < 3 billion. • Why do they seem to shut off? • Some may have to do with them no longer having gas to eat. • Also, as a black hole gets bigger, its density decreases. • If the density gets too low it no longer plays with its food, it eats it whole (although black holes are NEVER vacuum cleaners).
Active galaxies • The closest to quasars we get in the present is active galaxies. • These are galaxies that for some reason have materials falling into the center so that the center is bright. • Often times this is a result of a galactic merger. • 1% of galaxies are like this.
The light still travels out from the universe even after the quasar shuts off
What else we can learn from Quasars • Quasars are light bright search lights shining though our universe. • They shine not just through space but through time. • Since the universe expands, the light from the searchlight expands with time too. • This creates what is known as a Lyman-alpha forest • (called this because the Hydrogen emission it follows is called the Lyman-alpha line)
How Lyman-alpha forest works • Each time the light from the Quasar passes a cloud or galaxy the light from the Quasar is shifted to a different wavelength. • The gases in the cloud will emit and absorbed (based on the properties of the cloud or galaxy) at a specific wavelength that is not shifted. • So, each cloud adds its fingerprint or signature to the spectrum. • This allows us to know what everything is like along the line of sight (and in some way how the gas in the universe has evolved with time).
Conclusion • Some of the most distant observable objects are Quasars which are super massive and super bright black holes that are eating large amounts of matter.
