Galaxies & the Universe

Graph error bars

You often print graphs in which data points have error bars that are too short. About a third of the error bars don’t overlap the line of best fit. So which is wrong? Neither! When it comes to describing statistical uncertainty, such as drawing an error bar, the usual measure is plus or minus...

How many digits are satisfactory in the measurement of pi?

In the 3rd century BC, Archimedes proved that the ratio of a circle’s circumference to its diameter is less than 3 1/7 but larger than 3 10/71. That’s about 3.141. Later mathematicians have computed what we now call p (pi) to greater and greater accuracy — but how many digits are “enough”? The answer...

Should you use light-years or parsecs for astronomical distance?

You give astronomical distances beyond the solar system in light-years, but professional astronomy papers use parsecs. Which is preferable? Light-years, no question! Here’s how I see it. The parsec (which equals 3.26 light-years) is defined as the distance at which a star will show an annual parallax of one arcsecond. This means it is...

What would happen if I fed animatter into a black hole?

What would happen if I fed animatter into a black hole? Could this be a treatment to get rid of black holes? No. Antimatter has positive mass just like ordinary matter, so the black hole would merely get larger and heavier. Whatever fireworks happened inside the hole, if the anitmatter met up with ordinary...

What’s the density of the black hole in galaxy M87?

On page 20 of the March 2004 issue, you say the 3-billion-solar-mass black hole in the center of the galaxy M87 has an average density about that of air. I would love to understand how this makes any sense. The diameter of a black hole scales directly with its mass. That is, if you...

Where was the Big Bang located?

Where was the Big Bang located? In which constellation? Should we observe more young galaxies in that direction and more old ones in the opposite direction? You've got the commonest misconception about the Big Bang: that it happened at some particular spot in preexisting empty space, like an exploding hand grenade with galaxies for...

Why are the stars so far away?

Why are the stars so far away? That question has a real answer: Because if the stars were much closer, Earth wouldn’t exist, and we wouldn’t be here to ask. In places where stars are much closer together, such as globular clusters and the center of the Milky Way, planetary systems are probably unstable...