Page 42 - Curriculum Visions Dynamic Book

The Sun (with its planets that make up the Solar System) is one of these stars, lying near the edge of one of the spiral arms within the flat disk of our galaxy.
How stars move in a galaxy
The stars are not stationary in a galaxy but revolve around it. The time they take to move around
the center is extremely long—something like 200 million years. The fact that stars rotate around the center of the galaxy shows that they are bound inside it just as the planets are bound inside the Solar System. It shows that they are held together by the universal force of gravity.
How galaxies were formed
Because galaxies are the main features of the cosmos, understanding how they formed is key
to understanding much about how the cosmos works. Yet, so fundamental is this question and so remote the source of information, that the origin of galaxies is still shrouded in uncertainty.
One suggestion is that each galaxy must have begun through the rapid collapse of a single large gas cloud. In this cloud some stars emerged quickly, perhaps in the first 100 million years, while others formed more slowly.
An alternative suggestion is that a galaxy builds slowly by the clumping together of material.
Stars produce light when hydrogen burns to helium (see page 23). Helium now makes up about a quarter of the mass of our galaxy. So, it is possible to figure out the amount of stellar (star) matter needed to produce it. But that turns out to be much too small to account for all of the helium in our galaxy. So, although helium is produced by stars, most of the helium was not made in stars and must have been made much earlier.
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Edwin Powell Hubble (1889–1953)
Hubble is recognized as one of the great 20th-century astronomers and one of the first to think about an expanding Universe.
His observations at Mount Wilson Observatory provided the first evidence that nebUlae exist beyond the Milky Way.
Hubble began to classify galaxies into spirals, ellipticals, and irregulars (see right). He proved that galaxies were more or less evenly distributed in space for as far as could be seen. This supported the idea, known as the Cosmological Principle, that the Universe is uniform.
It was while making this classification that Hubble discovered that all galaxies are moving away from one another, that is, that the Universe is expanding.
He then found that the Universe was expanding in such a way that the ratio of the speed of the galaxies to their distance is a constant. This idea is known as Hubble’s Law, and the constant became known as Hubble’s constant, H. It is a cornerstone of modern astronomy (see page 53).
The value of H is between 15
and 30 km per second per 1,000,000 light-years. By using this constant,
it is possible to take the inverse (reciprocal) and deduce that the time since the start of the Universe (see page 54) is 10 billion to 20 billion years.