This week all eyes have been on the dwarf planet Pluto, as the New Horizons mission made its historic fly-past of this distant outpost of our solar system.
Until now, Pluto was just a tiny point of light in most telescopes, and at most a blurry blob in the largest telescopes such as Hubble. It is far too faint to pick out at all with my modest telescope. This is not surprising given that it is a small object at the other end of the solar system that generates no light source of its own, it simply reflects the sunlight that reaches its surface.
But of course, not all objects in the sky that seem faint are tiny inactive bodies. Common sense tells us that an intrinsically very bright object, such as a star or galaxy, will still appear faint to us if it is very far away.
A quasar, for example, is an incredibly luminous object, emitting the same amount of light as several hundred billion stars from a region only about the size of the solar system, but quasars are so far away from Earth (billions of light years!) that to us they appear no brighter than Pluto (which is 'only' 13 light hours away).
Quasars were discovered in the late 1950's and are still rather mysterious objects. They are compact points of light, so cannot be galaxies, yet neither are they stars, as the light spectrum of a quasar is very different from that of a star. They are very rare and are only found at great distances of billions of light years. They are therefore very old - the light that we see from them was emitted billions of years ago, when the universe was still young – so in fact they offer a fascinating glimpse far back into the history of the cosmos
The fact that they are so old, and that no nearer (and therefore younger) ones have been found suggests that the process that formed them only occurred in during a limited period when the universe was young. And their rarity suggests that this process was relatively short-lived. So what could this process be? Astronomers now believe that quasars are an early stage in the life cycle of supermassive black holes at the heart of early galaxies. An extremely luminous black hole sounds like a contradiction – but quasars are thought to be so bright beause of the sheer enormous mass of matter being heated up as it was consumed by the black hole. This energy was so great that the edges of the black hole started to glow and the radiation could escape in bright jets.
The source of all this matter was clouds of gas that were present in early galaxies. Of course, eventually all the matter near to the black hole would eventually be consumed, so as the quasar ran out of fuel, it would simply cease to glow – explaining why quasars seem to have been short-lived.
But recent discoveries suggest that it may not be quite as straightforward as this. Because of their age, their distance from us and their short life-span, quasars are very rare, so you would not expect to find several close together. Yet several groups of three quasars have been spotted, and in May this year researchers announced the discovery of a quartet of quasars within a massive cloud of gas – as their press release puts it, this seems as likely winning a lottery jackpot four times in a row! The chances are that this grouping is not a coincidence, in which case astronomers may have to rethink ideas about how quasars are formed. And this rethink could have profound implications for wider ideas about the way in which the early universe formed.
Interesting and potentially ground-breaking stuff!
Quasar quartet puzzles scientists http://www.mpg.de/9229786/quasar-quartet?filter_order=L&research_topic=
Black Holes and the Quasar connection http://www.spacetelescope.org/science/black_holes/
Hubble finds ghosts of quasars past https://www.spacetelescope.org/news/heic150