Intro to Astrophysics

Intro to Astronomy

Chapter 1

1. The Scale of the Universe

Our solar system belongs to the huge disk shaped collection of stars called the Milky Way Galaxy

Galaxy: is a great island of stars in space, held together by gravity and orbiting a common center.

The Milky Way is a large galaxy containing more than 100 billion of stars. It is one of the two largest amongst more than 70 galaxies, most of them small making them Local Group.

Groups of galaxies with many more large members are often called galaxy clusters.

Our solar system is located a little over halfway from the galactic center to the edge of the galactic disk.

The regions in which galaxies and galaxy clusters are more tightly packed are called superclusters, which are essentially clusters of galaxy clusters.

Our Local Group is located on the outskirts of the Local Supercluster

All of these structure make up our universe.

The universe: is the sum of total of all matter and energy, encompassing the superclusters and voids of everything within them.

Astronomical Distance Measurements

Astronomical Unit (AU): Commonly describe distances within our solar system in AU. Earths average distance from the Sun is 150 million kilometers.

Light-year (ly): the distance that light can travel in 1 year, which is about 10 trillion kilometers. Generally use light year to describe distances between stars and galaxies. It is a unit of distance not of time!

Light travels at the speed of light, which is 300,000 kilometers per second. In one second: 300,000 kilometers, in one year: 10 trillion kilometer.

One light year is equivalent to 9.46 trillion kilometers.

Looking back in time

Light takes a little of 1 second to reach Earth from the Moon, and about 8 minutes to reach Earth from the Sun.

Sirius the brightest star in the night sky, which is located about 8 light years away, because it takes light 8 years to travel this distance.

The Orion Nebula is a giant cloud in which stars and planets are forming. It is located 1350 light years from Earth, which is 1350 years ago. If any major events occur in the Orion Nebula since that time we cannot yet know about them because the light from these events has not yet reached us.

The farther away we look in distance, the further back we look in time.

The Andromeda Galaxy is about 2.5 million light years away which means we see it as it looked 2.5 million years ago

Basic Astronomical Definitions:

Star: A large glowing ball of gas that generates heat and light through nuclear fusion its core. Our Sun is a star.

Planet: A moderately large object that orbits the star and shines primarily by reflecting light from its star. According to the current definition, an object can only be considered a planet only if (1) orbits a star (2) is large enough for its own gravity to make it round (3) has cleared most other objects from its orbital path. An object that meets the first 2 criteria but has not cleared a path like Pluto is designated a dwarf planet.

Moon (or satellite): An object that orbits a planet. The term satellite is also used more generally to refer to any object orbiting another object

Asteroid: A relatively small and rocky object that orbits a star

Comet: A relatively small and ice-rich object that orbits a star.

Small solar system body: An asteroid, a comet, or other objects that orbits a star but it too small to qualify as a planet or dwarf planet.

Collections of Astronomical Objects

Solar system: The Sun and all the material that orbits it, including planets, dwarf planets, and small solar system bodies. Although the term solar system technically refers only to our own star system (solar means “of the Sun”) it is often applied to other star systems as well.

Star system: A star (sometimes more than one star) and any planets and other material orbits it.

Galaxy: A great island of stars in space, all held together by gravity and orbiting a common center, with a total mass equivalent to millions, billions or even trillions of stars.

Cluster of galaxies (or group of galaxies): A collection of galaxies bound together by gravity. Small collections (up to a few dozen galaxies) are generally called groups, while larger collections are called clusters.

Supercluster: A gigantic region of space in which many groups and clusters of galaxies are packed more closely together than elsewhere in the universe.

Universe (or cosmos): The sum total of all matter and energy that is, all galaxies and everything between them.

Observable universe: The portion of the entire universe that can be seen from Earth, at least in principle. The observable universe is probably only a tiny portion of the entire universe.

Astronomical Distance Units

Astronomical unit (AU): The average distance between the Earth and the Sun, what is about 150 trillion kilometres More technically, 1 AU is the length of the semimajor axis of Earth’s orbit.

Light-year: The distance that light can travel in 1 year which is about 10 trillion kilometers (more precisely 9.46 trillion km)

Terms Relating to Motion

Rotation: The spinning of an object around its axis. For Ex: Earth rotates once each day around its axis, which is an imaginary line connecting the North and South Poles.

Orbit (or revolution): The orbital motion of one object around another due to gravity. For example, each orbits the Sun once each year.

Expansion (of the universe): The increase in the average distance between galaxies as time progresses.

The Observable Universe

The measured age of the universe is about 14 billion years. If we look at a galaxy that is 7 billion light years ago- which means we see it as it was when the universe was half its current age. If we look at it from 12 billion light years away, we see it as it was 12 billion years ago, which when the universe was only 2 billion years ago.

The distance of 14 billion light years therefore marks the boundary (or horizon) of observable universe –the portion of the entire universe. We cannot see beyond that since its light has not reached us yet.

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