planets
what is the smallest and largest planet in our solar system?
Mercury is the smallest while Jupiter is the largest
why do comets have tails?
Comets develop tails as they approach perihelion, the place in their orbits when they are closest to the Sun.
how do black holes form?
Stellar-mass black holes are born when very massive stars explode in supernovae.
what is a star
A star is a luminous ball of gas, mostly hydrogen and helium, held together by its own gravity
Is there a center of the universe?
no, the universe began as a singularity (an infinitely tiny point with infinite density) that started expanding at the moment of the Big Bang.
Will there ever be a moment when all eight major planets are in a straight line on the same side of the Sun?
they can never line up in three dimensions because their orbital planes are all slightly different
where do comets come from?
Comets spend most of their lives far away from the Sun in the distant reaches of the solar system. They primarily originate from two regions: the Kuiper Belt, and the Oort Cloud. The Kuiper Belt is a disk composed mainly of icy bodies that stretches from about Neptune's orbit, out to about 50 AU from the Sun. The Oort Cloud is at the edges of the Sun's gravitational influence
how big are black holes?
Black holes are singularities: points of infinitely small volume with infinite density
what are constellations?
Since ancient times, civilizations have assigned constellations based on conspicuous patterns of stars in the night sky. These patterns were useful for astrological predictions, navigation, as well as for communication among astronomers.
How do we measure a galaxy’s distance?
a galaxy’s distance by measuring its redshift
Is there another planet whose moon(s) would fit perfectly over the Sun?
yes,35 other moons can hide the Sun’s disk completely as seen from their planet’s surface.
Why are comets and asteroids discovered when they are farthest from the Sun?
Objects that orbit the Sun spend most of their time near the farthest point of their orbits, where they move most slowly
what is a black hole
A black hole is a region of space where the force of gravity is so strong that the escape velocity exceeds the speed of light. Nothing, not even light, can escape its pull.
how far away is the closest star?
the Sun is the closest star to Earth, about 93 million miles away
What came before the big bang?
There was no before the Big Bang, the Big Bang created both time and space as we know it
Why is it that Saturn’s moon Titan has a dense atmosphere, yet Jupiter’s Ganymede and Callisto (about the same size as Titan) do not?
All three moons are a mixture of rocky materials and water ice, and the crystalline structure of ice is particularly good at trapping atoms of gas. But the outward characteristics of these moons are rather different, having been determined by the conditions near each planet as it formed.
what are steroids and where do they come from?
Asteroids are rocky objects primarily found in the asteroid belt, a region of the solar system that lies more than 2 ½ times as far from the Sun as Earth does, between the orbits of Mars and Jupiter.
HOW FAST DOES A BLACK HOLE SPIN?
When astronomers measure a black hole’s spin, they report the value as a fraction of the maximum allowed spin.
What is the brightest star in the sky?
The brightest star in the sky is Sirius, also known as the “Dog Star” or, more officially, Alpha Canis Majoris, for its position in the constellation Canis Major.
How did the universe begin? What happened during the Big Bang?
The Big Bang marked the origin of the universe, the beginning of its expansion from a singularity (or something close to a singularity), a single point that was infinitely small, infinitely hot, and infinitely dense.
Why do the outer, gas-giant planets rotate faster than the inner, terrestrial planets?
must have accumulated most of their mass from gas in the surrounding solar nebula. That gas formed individual spinning disks (from which many satellites formed), and most likely it carried a lot of angular momentum as it fell onto the outer planets' cores, causing them to spin faster and faster as they coalesced.
how big are asteroids?
It turns out that the asteroid must be no bigger than 3.9 v(ρe / ρ) kilometers in diameter, where ρ is the asteroid’s density and ρe is that of Earth in the same units (S&T: July 1984, page 62).
what is the density of a black hole?
The diameter of a black hole scales directly with its mass. That is, if you double the mass, you double the hole’s diameter. But if you double the diameter of something, you increase its volume by 2^3, or 8 times. So with twice the mass but eight times the volume, you’ve reduced the average density of your black hole by dumping stuff into it.
how long do stars live? how do stars die?
A star’s life expectancy depends on its mass. Generally, the more massive the star, the faster it burns up its fuel supply, and the shorter its life. The most massive stars quickly exhaust their fuel supply and explode in core-collapse supernovae, some of the most energetic explosions in the universe.
what is dark matter?
Dark matter is a mysterious type of matter that doesn't interact with any form of electromagnetic radiation, i.e., light. It doesn’t radiate light, and light doesn’t bounce off it.