Today's Object of the Day is a small blue object, nominated by paulrogers:587727226234208452
So let's see, what is it?
Small, compact and blue. Blueness means a fair amount of heat.
Z, or redshift, is zero. So it's in our galaxy. Here's the spectrum:
I wondered what the very high emission peak along the pink dotted line was. There are two of those solo pink dotted lines and I had a feeling they were probably something to warn us that any strong lines there didn't mean much - especially since no letters go alongside the lines. (The letters are either Fraunhoffer's labels
, which often mean atoms or molecules, or they're known atoms and molecules such as OIII
). It does look like it's atmospheric interference rather than a real emission line (especially as the green line underneath it is also high, which warns us of high error proability). Anyway, you can find that discussion here
, with quite a lot of input from EigenState
Now, I did not come across this while classifying. Neither, as far as I know, did Paul. It is not posted at least as this reference number anywhere on the forum. I found it via a database Stellar190 showed me.
I had one thing to go by: the name of the object. That is SDSS J010657.39 -- 100003.3 - as catchy as all astronomical names tend to be.
I went along to NED
, which is one of the databases which the SDSS pages often link to and which will tell us whether a galaxy is a known NGC or ARP or what have you. There is a Search box - but it told me "The object name that you submitted is not currently recognized by the NED name interpreter."
But NED is not the only database; there is also MAST
. (Which sounds like it should be Old Weather, but there you go - GANDALF was used
to look for peas rather than ring galaxies, after all . . .)
Give it a go now. Go into MAST - link above - and type SDSS J010657.39 -- 100003.3 into the Search box at the top. Leave the "Resolver" selected as SIMBAD, and tick all the boxes below - those show you what type of light
you want to search in. (SDSS uses infra-red, optical and ultra-violet. But it's worth looking everywhere.)
It took me back to NED but gave me the ra and dec of this object
. You can then go into any random SDSS page (such as this one
) and go to the top left hand corner where you can put the ra and dec into a box. And there was this object! Thank you Stellar190 for showing it to me.
So why did paulrogers nominate this and why did I go to quite a lot of trouble (because I made several mistakes with MAST and it became very surly and uncooperative) to get at this little blue object?
Because it's just been in the news
This object is two small white dwarfs in a binary system - with the shortest orbit ever known. They orbit each other every 39 minutes! Even Hubble takes an hour and a half to go round the Earth.
They are also slowly coming closer and closer together. In a few million years, they will merge.
Now what happens when a white dwarf gathers extra mass? It depends how big it is. Chandrasekhar
or "Chandra", in the 1930s, was much mocked by the great Eddington
for his calculations that there would be a point when gravity was strong enough to overcome electron degeneracy (the repulsion of electrons which stops matter collapsing, and keeps white dwarfs from getting smaller). One of his colleagues, Edward Arthur Milne
, asked him a difficult question: what would happen if a white dwarf was almost heavy enough to undergo this collapse, and then acquired just a tiny speck of dust which made it heavy enough? Apparently Chandra kept being asked this as a means of trying to prove him wrong. (He took the whole dispute very badly and there's some disagreement over whether Eddington was overbearing or whether Chandra was oversensitive. But anyway, the idea of a black hole was not talked about very seriously for many years.)
Actually Milne asked a great question and we now know that the answer is a Type 1a Supernova
(types b and c
being caused by core collapse of stars). And the great thing about Type 1a Supernovae is that they are all the same brightness. Why? Because all white dwarfs collapse at the same weight. The laws of physics are equal everywhere. They collapse at what is now known as the Chandrasekhar limit
- about 1.4 times the mass of our Sun. And being the same brightness, they're very useful as "standard candles" - a way to estimate distances.
But this is not the fate that awaits these white dwarfs. Their combined weights will not reach the Chandrasekhar limit. In fact, they only add up to about 50% of the mass of the Sun. There will be no supernova. Something else very strange will happen: they will become a single star and start to shine again.
Alone, each has run out of fuel in its core, but combined, and larger, they will be able to undergo nuclear fusion again - the larger a star, the better it is at doing this, because the hotter its core can get. Incidentally, the article describes the white dwarves as being made of helium, but I can't resist being pedantic and pointing out that a star does not become a white dwarf when its entire supply of hydrogen is exhausted - only when its core
has run out of hydrogen. Another Chandra-discovered limit, this called the Schönberg-Chandrasekhar
limit, is the size the core must reach when the star cannot continue to burn. It's generally about 10% of the mass of the star, though it varies as a proportion with the star's size. So in this case, 90% of the material can still be hydrogen. (If a star is very small, however, its layers can mix, and this means that more of its hydrogen would get burnt up. It's possible that this has happened - these white dwarves are pretty small. Anyway, I've gone off course - excuse me.)
In any case, a star can burn more than hydrogen. The discoverers of these white dwarves believe that they will become a helium-burning star. Helium burning tends to be the triple-alpha proces
(an alpha particle being a helium nucleus and also alpha particles from radioactivity, if you're interested). The triple-alpha process is actually such an interesting story that I'm going to leave it for another Object of the Day - but it involves our very existence in an extremely special way.
Look out for a new star in 37 million years!
The paper can be found here
Finally - I can't resist - because this is astronomy, have some more eye candy: a pretty White Dwarf picture from Universe Today, an article
about a star's planets continuing to orbit a white dwarf. I must say if these two have planets I would love
to meet them.