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ASTA02H3 (1)
Chapter 17

Chapter 17 Review: The Stars

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Department
Astronomy
Course Code
ASTA02H3
Professor
Brian Wilson

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January 11th, 2011
ASTA02
Intro to Astronomy and Astrophysics Part II
Chapter Seventeen: The Stars
The Solar Neighbourhood
Milky Way is our galaxy
oGalaxy: Enormous collection of stars and interstellar matter held
together by gravity
As with the planets, knowing the distance to the stars within the Milky Way is
essential to determining many of their other properties
By observing other distant galaxies, we can statistically infer the properties of
trillions more
oThe observable universe probably contains several tens of sextillions (1
sextillion = ) stars
Yet remarkable, despite their incredible numbers, the essential
properties of stars their interactions with their environment
can be understood in terms of just a few physical stellar
quantities: luminosity (brightness), temperature (colour),
chemical composition, size and mass
As more stellar distances become known, new insights into stellar properties
are obtained, and these in turn present new techniques for distance
measurement, applicable to even greater distances
Stellar Parallax
Parallax: Apparent shift of a foreground (essentially the closer one to the
observer) object relative to some distant background as the observers point
of view changes
oTo determine an objects parallax, we observe it from either end of
some baseline and measure the angle through which the line of sight
to the object shifts
www.notesolution.com
As the distance to the object increases the parallax becomes
smaller and therefore harder to measure
Any baseline on Earth is still too small to measure the
closest stars; however, photographs taken at different
times of the year are used instead
oUsing the diameter of Earths orbit around the Sun
as a baseline (about 2 AU)
Parsec: The distance at which a star must lie in order for its observed parallax to be
exactly 1 (arc second); roughly 3.3 light-years
oBecause parallax decreases as distance increases, we can relate a stars
parallax to its distance by the following formula:
Parallax distance (in parsecs) =    
oThus a star with a measured parallax of 1 lies at a distance of 1 parsec from
the Sun
Our Nearest Neighbours
Proxima Centauri is the closest star to the Earth (besides the Sun obviously)
oMember of a triple star system (three separate stars orbiting one another,
bound together by gravity) known as the Alpha Centauri complex
oProxima Centauri displays the largest known stellar parallax, 0.77, which
means that it is about 1/0.77 = 1.3 parsecs away approximately 270,000 AU,
or 4.3 light-years away
This is a fairly typical interstellar distance in the Milky Way Galaxy
If Earth were a grain of sand orbiting a marble-sized Sun at a
distance of 1 meter; the nearest star (Proxima Centauri), also a
marble-sized object, is then more than 270 kilometers away
oNothing else of consequence (except the other planets of
our solar system )exists in the 270 km separating the
two stars
Such is the void of interstellar space
www.notesolution.com
The next closest star called Barnards star has a parallax of 0.55 (1.8
parsecs or 6.0 light-years) and would be 370 km away from Earth in our
model
Stellar Motion
In addition to the apparent motion caused by parallax, stars have real spatial
(relating to, occupying, or having character of space) motion through the
galaxy
However, relative to our Sun that is, as seen by astronomers on Earth as we
travel through space along with our parent star stellar motion has two
components:
oA stars radial velocity along the line of sight can be measured using the
Doppler effect
oFor many nearby stars, their transverse velocity perpendicular to our line of
sight can also be determined by careful monitoring of the stars position in
the sky
Imagine Two pictures of the sky around Barnards star are taken 22-years
apart from one another
oWhen the pictures are superimposed on one another:
It is clear that Barnards star has moved within this 22-year
interval
The images of the other stars in the field of view coincide (are in
the same positions in both pictures, despite the time interval),
but those of Bernards star are not (they are in different
positions in either picture)
Because Earth was at the same point in its orbit around
the Sun when these photos were taken, the observed
displacement is not the result of parallax caused by
Earths motion around the Sun
oInstead, it indicates real spatial motion of Barnards
star relative to the Sun
The annual movement of a star across the sky, as seen from Earth and
corrected for parallax, is called proper motion
www.notesolution.com

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Description
January 11 , 2011 ASTA02 Intro to Astronomy and Astrophysics Part II Chapter Seventeen: The Stars The Solar Neighbourhood Milky Way is our galaxy o Galaxy: Enormous collection of stars and interstellar matter held together by gravity As with the planets, knowing the distance to the stars within the Milky Way is essential to determining many of their other properties By observing other distant galaxies, we can statistically infer the properties of trillions more o The observable universe probably contains several tens of sextillions (1 sextillion = ) stars Yet remarkable, despite their incredible numbers, the essential properties of stars their interactions with their environment can be understood in terms of just a few physical stellar quantities: luminosity (brightness), temperature (colour), chemical composition, size and mass As more stellar distances become known, new insights into stellar properties are obtained, and these in turn present new techniques for distance measurement, applicable to even greater distances Stellar Parallax Parallax: Apparent shift of a foreground (essentially the closer one to the observer) object relative to some distant background as the observers point of view changes o To determine an objects parallax, we observe it from either end of some baseline and measure the angle through which the line of sight to the object shifts www.notesolution.com As the distance to the object increases the parallax becomes smaller and therefore harder to measure Any baseline on Earth is still too small to measure the closest stars; however, photographs taken at different times of the year are used instead o Using the diameter of Earths orbit around the Sun as a baseline (about 2 AU) Parsec: The distance at which a star must lie in order for its observed parallax to be exactly 1 (arc second); roughly 3.3 light-years o Because parallax decreases as distance increases, we can relate a stars parallax to its distance by the following formula: Parallax distance (in parsecs) = o Thus a star with a measured parallax of 1 lies at a distance of 1 parsec from the Sun Our Nearest Neighbours Proxima Centauri is the closest star to the Earth (besides the Sun obviously) o Member of a triple star system (three separate stars orbiting one another, bound together by gravity) known as the Alpha Centauri complex o Proxima Centauri displays the largest known stellar parallax, 0.77, which means that it is about 10.77 = 1.3 parsecs away approximately 270,000 AU, or 4.3 light-years away This is a fairly typical interstellar distance in the Milky Way Galaxy If Earth were a grain of sand orbiting a marble-sized Sun at a distance of 1 meter; the nearest star (Proxima Centauri), also a marble-sized object, is then more than 270 kilometers away o Nothing else of consequence (except the other planets of our solar system )exists in the 270 km separating the two stars Such is the void of interstellar space www.notesolution.com
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