So you’ve just discovered a spiral galaxy with your brand new telescope. Well, it wasn’t you exactly that discovered it, but you have found a pretty picture the Hubble Space Telescope took of it on the internet. Here it is (it’s called M101):
Isn’t that nice?
Now galaxies are pretty similar to celebrities (bear with me, I’ve got a point, really) as once you’ve taken a picture of one, the next thing you want to know is how much mass it has (and whether it’s ever had an affair with Tiger Woods). Easy you think – just count up the mass from all the stars and you’re done. Except, like most things in life, it’s not that simple…
The problem is shown by something called a Rotation Curve – this plots the velocity that stars at different distances from the galaxy’s centre are orbiting round it. Since the majority of the stars are in the central bulge (look at the picture above – it’s brightest in the centre), then you’d expect that the orbital velocities would decrease as you got further out. (This happens in the Solar System – Earth is going round the Sun faster than things further out, like Neptune.) However, when the first Rotation Curves were plotted they were flat, implying that the orbital velocities are the same in the outer regions as they are closer in. This is neatly summed up by this illustration I found at AstronomyOnline:
This suggests that there must be a lot more mass in the galaxy, invisible mass that’s not in the form of stars or clouds of gas or dust, which extends far beyond its visible edges, and can only be detected by its gravitational effect on the ‘normal’ matter its enveloping. This is dark matter, and it looks like it makes up the majority of the mass of the Universe. Which is embarrassing – no scientist likes to admit that they don’t really know what most of the Universe is made of.
Stop by tomorrow to find out what we think we know about this mysterious stuff and what it could be.