Intro to Astrophysics

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Light year is a unit of distance not time!

The Scale of the Solar System

Voyage Scale: shows the sun and the planets at their correct sizes not distance

The sun is the size of a grapefruit, Jupiter about the size of a marble, and Earth is about the size of the ball point in pen.

Common misconception:

Our solar system is a single star system while our galaxy is a collection of more than 100 billion star systems. Our galaxy is 100 million times larger in diameter than our solar system.

Order of magnitude estimates: Estimates good to about the nearest power of 10

Sand on all the beaches earth is comparable to the number of stars

1.2 The History of the Universe

The Big Ban, expansion, and the Age of the universe

Telescopic observations of distant galaxies show that the entire universe is expanding meaning that average distances between galaxies are increasing with time. This fact implies that galaxies must have been closer together in the past, and if we go back far enough, we must reach the point at which the expansions began. We call this the Big Bang.

The universe has a whole continues to expand, individual galaxies and galaxy clusters do not expand.

Stellar Lives and Galactic Recycling

Stars are not living organisms, but go through life cycles.

A star is born when gravity compresses the material in a cloud to the point which the center becomes dense enough and hot enough to generate energy by nuclear fusion the process in which lightweight atomic nuclei smash together and stick (or fuse) to make heavier nuclei.

In its final death throes, a star blows much of its content back out into space. The most massive star dies titanic explosions called supernovae. The returned matter mixes with other matter floating between the stars in the galaxy, eventually becoming part of a new clouds of gas and dust from which new generations of stars can be born. Galaxies therefore functions as cosmic recycling plants, recycling material expelled from dying stars into new generations of stars and planets.

Our own solar system is a product of many generations of recycling.

Star Stuff

By studying the early universe contained only simple chemicals: hydrogen and helium and a trace of lithium. We and earth are made primarily in other elements: such as carbon, nitrogen, oxygen, and iron.

Other elements, were manufactured by stars, some through nuclear fusion that makes stars shine and others through nuclear reactions accompanying the explosions that end stellar lives.

By the time our solar system formed about 4 (1/2) billion years ago, earlier generations of stars had already converted up to 2% of our galaxy’s original hydrogen and helium into heavier elements. Therefore, the cloud that gave birth to our solar system was made roughly 98% hydrogen and helium and 2 % other elements. This 2% was small enough to make the small rocky planets of our solar system including Earth.

How do our lifetime compare to the age of the universe?

On this cosmic calendar, the Big bang occurred at the first instant of January 1st and the present is stroke of midnight on December 31st

On this time scale, the Milky Way Galaxy probably formed in February

Our solar system and our planet did not form until early September on this scale (4 ½ billion years ago in real time). By late September, life on Earth has been flourishing. On the scale of the cosmic calendar, recognizable animals became prominent only in mid-December. Early dinosaurs appeared on the day after Christmas. Some 60 million years later or around 9 pm on December 31st of the cosmic calendar, early hominids began to walk upright.

1.3 Spaceship Earth

Rotation and Orbit

Earth rotates once each day around its axis, which is the imaginary line connecting North and South Pole. Earth rotates from west to east, counterclockwise as viewed from North to the South Pole. Which is why the Sun appears to rise in the east and set in the west each day

We are racing around the Sun at a speed of 100,000 kilometres per hour (60,000 miles per hour)

The Earth’s orbital path defines a flat plane that we call the ecliptic plane

Earths axis is titled 23 ½ “ from a line perpendicular to the ecliptic plane.

This axis tilt happens to be oriented so that the axis points almost directly at a star called Polaris or the North Star.

The average Earth – Sun distance is 1 AU (150 million kilometers)

Motion within the Local Solar neighborhood

Stars in the local solar neighborhood move randomly relative to one another at a typical speeds of 70,000 km/hr

On average our Sun is moving relative to nearby stars at a speed of about 700,000 kilometers per hour (40,000 miles per hour) almost 3x times as fast as the Space Station orbits Earth.

Galactic Rotation

Our solar system located about 27,000 light years from the galactic center, completes one orbit of the galaxy in about 230 million years. By calculating the speed it is close to 800,000 kilometers per hour

Stars at different distances from the galactic center orbit at different speeds and we can learn how mass is distributed in the galaxy by measuring different speeds. Such studies indicate that the stars in the disk of the galaxy represents only the “tip of the iceberg” compared to the mass of the entire galaxy. Most of the mass of the galaxy seems to be located outside the visible disk but the matter that makes up this mass is completely invisible to the telescope. The nature of this matter is referred to as dark matter (because of the lack of light from it). Studies of other galaxies indicate that they also are made mostly of dark matter, which means this mysterious