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Astrophysics

Year 11 Physics - Astrophysics
by

DJ Horvat

on 6 March 2016

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Transcript of Astrophysics

Astrophysics is a branch of astronomy that deals with fundamental laws (nature) of the universe. They study the interactions and behaviours between celestial objects such as galaxies, stars, cosmos, etc
Astrophysics
An astrophysicists main goal is to understand how the universe works.
Astrophysics is the application of physics to the study of the universe.
The

universe

is everything,
including all matter, all space
and all time.
In a sense it all began with Galileo pointing his telescope at the heavens in 1609, only to find that they bore an unexpected resemblance to the Earth.
The laws of physics that he had begun to formulate would not only work down here, but out there as well; we were a part of the universe.
Apart from the stars and planets, other objects are visible in the night sky, including a bright band known as the Milky Way. Galileo’s telescopic investigations showed this to be an enormous concentration of stars too faint for the naked eye to distinguish.
The discovery of galaxies
A
constellation
is a group of
stars forming recognisable
patterns in the sky.
A
nebula
(plural = nebulae) is a
vast cloud of gas in space.
Orion Nebula
This is an optical image showing part of
the Milky Way.
It shows an area extremely rich in star clusters and
dark and bright (pink) nebulae.
Dark nebulae are formed by clouds
of interstellar dust which scatter
and absorb the radiation from
stars situated behind them.
Bright nebulae are clouds of interstellar gas which is ionised and lit up by the ultraviolet radiation of nearby hot and young stars.
This is the Large Magellanic Cloud Nebula.
William Herschel and his sister Caroline Hershel shook the ancient perceptions of the universe when William Herschel built the largest telescope of his time and made some very significant discoveries
He also noticed that some stars, called binary stars, occurred in pairs that orbited a common point. When he analysed their orbits he found that they obeyed Isaac Newton’s law of gravitation that described how masses attract each other.
Herschel also found that the Sun, with its planets, was moving through space. This meant that the Sun, like the Earth, was not not fixed at the centre of the universe.
Moreover, the stars did not seem to be evenly distributed throughout space but were gathered together in an elongated ‘island’ of stars.
Some of the nebulae the Herschels were seeing were actually galaxies.
A
galaxy
is an enormous group
of stars held together by gravity.

One light-year is the distance that light travels through a vacuum in one year
1 lightyear = 9460000000000000 m
In the 1920s and 1930s Hubble and other astronomers surveyed the sky in search of galaxies and they found tens of thousands that were roughly evenly distributed throughout the sky.
In the 1960s, astronomers conducted more detailed surveys of galaxies, mapping over one million. These maps showed that not only were stars clumped into galaxies, but galaxies were clumped into clusters.
Today estimates suggest that there are a hundred billion galaxies in the universe, each containing millions, if not billions or hundreds of billions, of stars. The size of the universe is beyond comprehension.
The universe is in fact so large that if we were anywhere else, even within our own galaxy, the Earth would barely feature as a speck of dust worthy of investigation.
Before you can even begin to understand the size of the universe we need to know about
light years
...
He is most famous for his discovery of a new planet, Uranus, in 1781. This planet was so far away that its discovery doubled the size of the known solar system.
The more astronomers have learnt about the universe, the less significant out place in it can seem.Yet it mat have also heightened our sense of how extraordinary Earth and its inhabitants really are.
Measuring distances to stars and galaxies
The Andromeda nebula is an enormous galaxy of stars. It is actually a galaxy much bigger than the Milky Way, and is one of the closest to it at a distance of over two million light-years.
How do we know this?
Parallax is an effect that is noticed whenever an object is viewed from two different locations.
Parallax is the only direct method we have of measuring distances in space; however, this method is quite limited. Variation in the refraction of light through the atmosphere (like heat haze) is one of the reasons that it
is difficult to measure the parallax of all but the nearest stars.
The Hubble Space Telescope, in orbit outside
the Earth’s atmosphere, avoids this problem and has made a significant contribution to measuring the distances of our local stars.
This method of measuring the distance to stars is known as stellar parallax.
This leads to a natural unit for the distance of stars, the parsec. The parsec is the distance to a star that shows a parallax angle of 1 arc second.
Arcsec - To measure the small angles between stars 1° is divided into 60 ‘arc minutes’, and the arc minute (arcmin) divided again into 60 ‘arc seconds’. So the arc second (arcsec) is only 1/3600 of a degree.
1 pc = 206265 AU
1 AU = 149600000000 m
Question
The parallax angle of the star Sirius is 0.379 seconds of arc. How far is Sirius from
Earth in:
(a) parsecs
(b) meters
(c) light-years?
Star brightness and distance
The luminosity of a star is the total energy that the star radiates per second.
As this energy moves away from the star at the speed of light, it becomes spread over a sphere of surface area 4πr^2, where r is the distance to the centre
of the star.
The brightness of a star is defined as the energy per second per square metre at the place where the star is observed.
The apparent magnitude scale is a
concoction of an ancient Greek scale with a 19th
century mathematical redefinition! Visible stars
range from –1.44 (Sirius A) ‘down’ to about +6
for stars barely visible under the best conditions.

a difference in magnitude of 5 corresponds exactly to a factor of 100 times in apparent brightness
Brightness and luminosity
When measured in this way the intrinsic brightness is
the luminosity (L) and is measured in watts.
While Sirius is the brightest star in the sky as we see it, Canopus is intrinsically 3000 times brighter. However, it is 36 times further away.
Clearly there must be a relationship between the apparent brightness of a star, its distance from us and its intrinsic brightness, the actual brightness
of the star.
Apparent Magnitude
The luminosity of a Cepheid variable star is known to be 8.0 × 10^29 W. Astronomers measure its brightness to be 8.6 × 10^-10 W /m^2. What is its distance from us in light years?
in 1912 Henrietta Leavitt (1868–1921) of Harvard University found that a Cepheid variable (a large pulsating star) can tell us how bright it is at
a fixed distance.
How do you determine the brightness of any star if you don't know its distance?
Leavitt noticed that the brightness of Cepheid variables was proportional to their period of pulsation. This was important because all of the stars in each galaxy are approximately the same distance from Earth.
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