Read more: 鈥�Mysteries of the Milky Way鈥�
V838 Monocerotis
In February 2002, this previously undistinguished star about 20,000 light years away briefly achieved a luminosity a million times that of our sun. The following month it happened again. And in April. It was first assumed to be a nova 鈥� a white dwarf that pulls gas off a companion until it triggers a thermonuclear explosion on its surface. But novas don鈥檛 happen three times in quick succession and then go quiet.
Was it a rarely seen flare-up near the end of a giant star鈥檚 life? The scream of two stars colliding? Or did one star swallow three giant planets? What is certain is that the triple burst of light was reflected off nearby dust to surround the object with rapidly changing light shells, making it a true cosmic beauty.
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Blue stragglers
The Milky Way鈥檚 dense globular clusters are spherical swarms of red, lightweight and ancient stars, most of them more than 10 billion years old. A few globular-cluster stars, however, shine in blue-white light 鈥� suggesting something anomalously hot, young and bright.
We now think these 鈥渂lue stragglers鈥� are just as old as their companions, but have somehow been rejuvenated. Some may have sucked gas from a neighbouring star, compressing their central nuclear engine to make them burn faster and hotter. Others may be the offspring of stellar mergers 鈥� two cool red stars fusing to make one hot blue one.
Sagittarius A*
Sagittarius A* is a source of radio waves at the Milky Way鈥檚 centre, thought to hold a huge black hole four million times the sun鈥檚 mass. In some galaxies, such a black hole would be a fearsome source of radiation, blazing in light and X-rays as it feasted on nearby gas.
Not so in our galaxy. That is partly because Sagittarius A* has a much scantier supply of gas, but even so it is faint, and seems unusually inefficient at converting gas into heat and light. Some clues as to why might come next year, when a nearby gas cloud looks set to plunge into our listless giant鈥檚 maw.
S2
S2 is a fast, intense, blue-white star that frankly has some explaining to do. It orbits within a whisker of the galaxy鈥檚 central black hole, Sagittarius A* (see 鈥淢ilky Way mysteries: Where are all the supernovae?鈥�), swinging by at a speed of up to 5000 kilometres per second, or nearly 2 per cent of the speed of light.
At this distance, the black hole鈥檚 gravity should shred gas clouds before they can condense into new stars. And although a star might migrate inwards from more tranquil breeding grounds, S2 is a bright young thing no more than about 10 million years old, whose lifetime seems too brief for such a trek.
SDSS J102915+172927
Most stars today contain a moderate hoard of heavy elements inherited from earlier stellar generations. Not so this one, more than 4000 light years away. , it is an almost pristine blend of hydrogen and helium, with just 0.00007 per cent other stuff.
That is similar to the primordial matter emerging from the big bang. Such pure gas, lacking the carbon and oxygen that normally help clouds to cool and condense, was thought to form only colossal, short-lived stars. No one knows how this anomalous object managed to form 鈥� perhaps it was a fragment spun off during the birth of a supergiant star, back in the dark ages of the universe.