Astronomy

How many galaxies are there?

I saw a couple of good posts on this from astronomycafe.net on their ask the astronomer page. The general consensus of all the answers out there seems to be that we do not know exactly, but there are at least a hundred billion, and possibly several hundred billion. The following is quoted from Astronomy Cafe:

We do not know exactly. Within the part of the universe we can observe there seem to be at least 100 billion, but this could be an underestimate if you include dwarf galaxies that are too far away to be easily seen by even the Hubble Space Telescope.

Imagine looking at a dime about 75 feet away. The Hubble Space Telescope can represent this narrow “keyhole” stretching to the visible horizon of the universe.  This “deep field image” shows hundreds of galaxies in a region only an arcminute across. What is an arcminute? Take the diameter of the full moon, divide it by 30 and that’s about an arcminute. While this is a tiny tiny view of the visible universe, it can give scientists a way to extrapolate and estimate how many galaxies there may be.

Now, even more interesting, the following answer comes from Kathy Wollard’s “How Come?” book. Our galaxy, the Milky Way is an immense spiral galaxy with about 200 billion stars. This number of stars is almost unimaginable to us, but even more astounding is that each star is often trillions of miles from its nearest neighbor star. This is how big ONE galaxy can be. Current theories are that these huge galaxies are probably the result of tens or hundreds of smaller galaxies colliding and becoming one. Of course, a “collision” in this sense actually can take millions of years. The basis of this theory is that when astronomers look far out into space, more than 2 billion light years (which is also looking 2 billion years into the past), they see more small galaxies and fewer big ones.

Imagine this: the light we see coming from a galaxy 2 billion light years away, is 2 billion years old. This is just hard to comprehend.

Who discovered Mars?

No one person is considered to have discovered Mars. As it is very bright in the night sky, it has been visible since the first humans gazed up to the heavens. What we do know is that it was named after the Roman god of war- presumably because of its red color which may have reminded our ancestors of blood.

1659: Christian Huygens discovered the dark spot located in the boundary between the northern lowlands and southern highlands of the planet. It was later called the Syrtis Major.

1877: Astronomer Giovanni Schiaparelli discovered what he believed to be several lines crossing one another. He claimed they were water canals made by intelligent creatures.

1877: Astronomer Asaph Hall spotted the two moons and named them Phobos and Deimos (fear and panic). He named them after the mythical horses that pulled the chariot of the Roman god, Mars.

1971: Mariner 9 returned images of Martian volcanoes and canyons. It discovered Olympus Mons, a massive volcano towering over 15 miles above the surface. Mariner 9 also found evidence that water once flowed on Mars. There were no sightings of Schiaparelli’s famous canals.

1975: Viking I and II spacecraft landed on Mars to study its surface. They analyzed the rocks and soil of the planet while providing us with information about its atmosphere and weather patterns.

Source: Wikipedia and the University Corporation for Atmospheric Research.

How dense is matter inside a black hole?

First, the simplest definition of density: it is how heavy something is relative to its size. A pound of rocks weighs the same as a pound of ping pong balls. But the ping pong balls take up a lot more space. Hence, the rocks are much more dense. Another way to look at density is to think of it as a measure of the “compactness” of matter.

More background… at the center of an atom is a very dense core called the nucleus. It’s composed of protons and neutrons (held very tightly together). Surrounding this nucleus in somewhat of a cloud are the electrons. Atomically speaking, the electrons are very far apart and far from the nucleus. Consider this: the entire atom composed of an electron cloud surrounding the nucleus is about 99.9% empty space.

The electrons are negatively charged and repel anything else negatively charged with a very strong electromagnetic force, or EMF. Now imagine a force strong enough to overcome this EMF and compress atoms to a much greater density. This is what happens in old and dying stars– the compressing force of gravity starts to overcome this electromagnetic force. The atoms start squeezing together resulting in what’s called degenerate matter. Stars involved in this process are called white dwarfs and the matter in them can reach a density of one million times that of water.

While this is very dense, it is not the densest state that matter can reach. If the dying star is massive enough, its gravitational force can be powerful enough to overcome the repelling force in the degenerate matter. The center of this body is now called neutronic fluid and these stars are now called neutron stars or pulsars. Now we’re getting pretty dense. A 1cm cube of neutron star material would weigh 100 million tons and if dropped would fall straight through to the center of the earth.

Now for even bigger stars (more than three times the mass of our sun), it can have a gravitational force strong enough to break down even this neutronic matter. After this, there will be no barrier left. The matter can not compress any further and it is basically a single point called a singularity. A star that has collapsed into itself to this point is called a black hole.

Since there is no way to measure anything of this magnitude, estimates are made by estimating the matter outside and near this singularity. If we use matter on Earth as a first order of magnitude, degenerate matter (inside white dwarfs) is about one million times as dense. Neutronium (inside neutron stars) is about one trillion times as dense. And finally, black holes, which are about ten trillion times as dense.

Source: Why Nothing Can Travel Faster than Light. Contemporary Books, 1993.

Why do stars twinkle?

Actually, the intensity of the stars themselves doesn’t fluctuate, rather it is the light they emit that appears to brighten and dim as it passes through the air in our earthly environment. Were you to look at the stars from an airless environment, say the moon, you would see them as solid luminous points of light.

So why does the air make them appear to twinkle? Basically, our atmospheric air is constantly moving, with warm air masses constantly rising and cool ones sinking. The densities of these masses is different and thus the refractive (light bending) characteristics are also different. So when starlight passes through thinner air, then thicker, then thinner again, it bends accordingly and appears to shimmer.

Now you may wonder, if light passing through our atmosphere refracts like this, why don’t planets seem to twinkle? It’s because the planets are much much closer to Earth than are the stars. We actually see planets as tiny disks rather than single points of light. The light from planets will still be bent, but since we see it as a “disk,” it is made up of many points of light. Each of them may brighten and dim as they pass through our atmosphere, but the average intensity of these points of light is fairly constant.

Source: How Come? by Kathy Wollard.

Where do comets come from?

Mathematical theory suggests that most comets may come to the solar system from very far away, as far away as 100,000 Astronomical Units. In this picture, the solar system is buried deep within the cloud.

An Astronomical Unit (or AU) is the distance from the earth to the sun and is equivalent to about 93,000,000 miles. Mars is 1.5 AU from the sun, Jupiter is 5 AU from the sun, and Pluto is 39 AU from the sun. So comets come from very far away indeed.

Comets are observed to come to the solar system from all directions, therefore the place where the comets come from is thought to be a giant sphere surrounding the solar system. This sphere is called the Oort cloud after Jan Oort who suggested its existence in 1950.

But some comets may come to the solar system from closer in. The place where these comets come from is called the Kuiper Belt, which is located past the orbit of Pluto. Source: the University Corporation for Atmospheric Research.

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What is Hubble’s Constant?

Hubble’s Constant is the rate ratio of the speed at which a galaxy is moving away from Earth divided by its distance from Earth. Note, this is obviously not our galaxy, but other galaxies in the Universe.