What are Quasars?

Discovering Quasars

A Very Strange Star3C 273 is a very

powerful radio source In 1963, Maarten Schmidt

took optical images of it and found it looked like a normal blue star (a blue point-source)

He also took a spectrum and found it had very odd spectral lines

Not a starCalled a Quasi-Stellar

Object (QSO) or Quasi-Stellar Radio Source (Quasar)

Let’s Try to Find the Quasar!

The Spectrum of a Quasar

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Comparing Quasars and Stars

What can we Learn from Quasars’ Spectra? How are they Different from stars’?

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• Remember: What are the (main) things that can be learned from an astronomical spectrum?

Quasars and Stars look quite different through different filters

• Spectra of quasars are very different from those of stars, so stars and quasars look quite different through different filters

Quasars Live In The Centers of Distant Galaxies

More than 500,000 quasars are known

Almost all quasars are farther than three billion light-years away

Galaxies hosting quasars often appear irregular, as if they recently merged with another galaxy

Changes in Light from Quasars Show That They’re far Smaller than Galaxies

Time of rise and fall in quasar brightness tells us the quasar’s maximum size

Quasars, In Brief…Quasars are at “astronomical” distances

Most are over 3 billion light years away, some back to distances corresponding to the early Universe

Great distances - must be very luminousThe most luminous quasars are (intrinsically) more

than 100 times the brightness of our GalaxyQuasars are highly variable on short timescales

Must be small..about the size of our Solar SystemAre at the centers of (often irregular) galaxiesSpectra suggest very, very hot gas rapidly orbiting

something

Accretion of gas onto a supermassive black hole is the only way to explain quasarsMaterial about equivalent to 1000 Suns is swallowed by the black hole in a quasar each year

As a piece of matter falls into the black hole, ~25% of its mass (E=mc2) is converted into energyCompare this to the fusion that fuels our Sun, for which ~1% of the mass is converted to energy

What is the Power Source for Quasars?

The inner structureof a quasar

Do Galaxies really have Supermassive Black Holes at their Centers?

This objectis in a region

that issmaller than

a neutronstar...only a supermassive

black hole can explain this

Orbits of stars near the

Galactic center prove there is

an object 4 million times the mass of the Sun at

the center of our Galaxy

Where are all the Quasars now?Supermassive black holes probably exist in

most, if not all, galaxies’ cores There are no really bright quasars nearby, i.e.,

at this point in time in the history of the UniverseQuasars were therefore more common in the

past that they are nowAs the Universe

aged, the quasarshave disappearedquasars have

turned off as the Universe aged

It’s Just a Phase They’re Going Through

All galaxies may have passed through a quasar-like stage earlier in time

Mergers between large galaxieschannel gas tothe black hole ata galaxy’s center,triggering a quasar phaseuntil the gas runs out Galaxy evolution from spiral to quasar to

elliptical galaxy via an interaction/merger

Why are Quasars Scientifically Interesting?

What are Quasars?