Question 1

What is a quasar?

Accepted Answer

A supermassive black hole with enough gas falling towards it that the gas itself becomes the brightest thing for millions of light years. Nothing is escaping the hole — the light comes from the disc outside it, heated by friction to the point where it radiates more than the whole surrounding galaxy.

Question 2

Why are they all so far away?

Accepted Answer

Because they are a phase rather than a kind of object, and the phase mostly happened early. Feeding at that rate needs gas delivered fast, which was common when galaxies were still colliding and settling; the same black holes are still there today, mostly quiet, and one of them is at the centre of this galaxy.

Question 3

How bright is that, exactly?

Accepted Answer

The brightest here would outshine the Milky Way by a factor of a thousand. Several are visible in an amateur telescope from billions of light years away, which nothing else in this catalogue can claim.

Question 4

Are quasars dangerous?

Accepted Answer

Not to anything at these distances, and the nearest is 600 million light years away. A quasar is a hazard only to the gas in its own galaxy, where the wind it drives can strip out the material stars would otherwise form from.

Question 5

How is the distance measured?

Accepted Answer

By parallax where the object is near enough — the shift in its apparent position as the Earth crosses its orbit, which is geometry and needs no assumptions. Beyond that, distance is inferred from how bright something of known luminosity appears, and the further out the ladder goes the more the uncertainty grows.

Question 6

How firm is the order?

Accepted Answer

Firmer at the top than at the bottom. Neighbouring rows often sit inside each other's error bars, so a swap between two adjacent entries is a revision rather than a correction. Each object's own page carries the figure with its uncertainty and the method it came from.

Question 7

Who worked out what they were?

Accepted Answer

Maarten Schmidt, in 1963, looking at 3C 273. It had been catalogued as a radio source with a starlike counterpart and a spectrum nobody could identify, until he recognised the lines as ordinary hydrogen shifted far towards the red — which put the object at a distance no star could be, and made it the most luminous thing then known.

Question 8

Why does the distance say a different number from the article I read?

Accepted Answer

Because there are two answers and they are both correct. Press coverage quotes how long the light has been travelling; the figure here is a luminosity distance, which is what the object's brightness implies once the expansion since is accounted for. Each object page carries both.

Question 9

Why do some of these vary so violently?

Accepted Answer

Because their jets point almost at us. A jet aimed at the observer is beamed: the same outburst looks far brighter and appears to run far faster than it would from the side, which is why a handful of entries here can double in a night from across the universe.

Question 10

Do the black holes have measured masses?

Accepted Answer

Some. Where a paper has measured one it is on the object's page; where nobody has, the field is empty rather than estimated. Most quasar masses come from the width of a single emission line and carry a factor of two or three, which is worth knowing before comparing two of them.

The quasars

COMMON QUESTIONS