Mon Oct 13, 2025 OLBERS AND HIS PARADOX
Heinrich Wilhelm Olbers, who discovered the asteroids Pallas and Vesta a couple of hundred years ago, was born on October 11, 1758. He is best known for what’s called, Olber’s Paradox. Olbers asked a simple question: "why is the sky dark at night?" Now that seems a bit silly - after all, the sky is dark at night because the earth rotates into its own shadow, what we call night. ”I know that,” he said. But if the universe is infinite in size, then that means there's an infinite number of stars out there. So no matter where you look, you'll eventually find a star - the sky should be ablaze with light! But it's not. This suggests that the Universe is perhaps not infinite, and that there was a definitive point in time in which everything began, and also that our Universe is expanding!
Tue Oct 14, 2025 HOW TO SEE A BLACK HOLE
In the autumn evening sky, there are three bright stars high overhead which are called the Summer Triangle. In the middle of this triangle there is a great mystery - something which is invisible to the eye - that enigmatic phenomenon known as a black hole. It is called Cygnus X-1, and we can't see it directly because its gravity field is so intense that light can't escape it. But we know that it is there, because we've discovered an incredible amount of x-rays pouring out of this part of the sky. Cygnus X-1 is part of a binary star system. Gas from its companion, a massive blue giant, is being pulled from it to feed the accretion disc surrounding the hole; it’s here that the x-rays are being made, just outside the black hole's event horizon - its point of no return; it’s about 2500 parsecs, or a little less than 48 quadrillion miles from Earth.
Wed Oct 15, 2025 FARTHEST NAKED-EYE OBJECT
What’s the farthest thing you can see without a telescope? Off in the northeastern sky this evening, you can find the answer to this question, but only if the skies are very clear, and very dark, and you know just where to look. It’s a small, very dim smudge of light that lies in the direction of the constellation Andromeda. But this little cloud is neither little, nor does it have any physical connection with the stars of Andromeda, which are merely trillions of miles away. It’s another galaxy, comprising hundreds of billions of stars – bigger than our own Milky Way - and it’s about 2 and a half million light years away. So if you ever catch a glimpse of this far-flung object, which we call the Andromeda galaxy, you’ll be looking at something that is 15 million trillion miles away – and that’s how far out your eye can see.
Thu Oct 16, 2025 CHANDRASEKHAR AND BLACK HOLES
Subrahmanyan Chandrasekhar, born on October 19th, 1910, was the astronomer who figured out just how massive a star had to be in order to turn into a black hole. If a star is one and a half to almost two and a half times more massive than our sun, when it dies, it explodes and becomes a supernova, then collapses to become a neutron star. But if a star has over 2.4 times the sun’s mass, the final gravitational collapse is so powerful that the star doesn’t blow up – it blows in to become a black hole! The imploding star shrinks down to a singularity, a point of ridiculously high density. We can’t see black holes directly, but we know that they are out there, because as their gravity pulls matter in, x-rays are released, which escape the hole’s event horizon, the point of no return.
Fri Oct 17, 2025 THE MOON AND VENUS TOGETHER IN PREDAWN
Last month many of us got to see a very pretty conjunction of the moon and the planet Venus before sunrise. The moon orbits the earth once every month, which means that we should be expecting another conjunction this weekend, when the two celestial objects appear to be right next to each other. Conjunctions are a bit of an illusion; while the moon and Venus may appear to be almost touching, there’s no danger of collision because the distance between the earth and the moon is only about 240,000 miles, while at the moment Venus is almost a hundred and fifty million miles away. But if you go outside over the next couple of mornings, just before sunrise, you can watch the moon coming closer to Venus, and on Sunday morning at dawn, the two will be together low near the eastern horizon!
