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Chapter 17

Chapter 17 Review: The Stars

Course Code
Brian Wilson

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January 11th, 2011
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 observer’s point
of view changes
oTo determine an object’s parallax, we observe it from either end of
some baseline and measure the angle through which the line of sight
to the object shifts

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As the distance to the object increases the parallax becomes
smaller and therefore harder to measure
β€’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 Earth’s 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 star’s
parallax to its distance by the following formula:
Parallax – distance (in parsecs) =    ξ€„ξ€‚ξ€…ξ€†ξ€‡ξ€†ξ€ˆξ€ˆξ€†ξ€‰ξ€Šξ€‹ξ€†ξ€‡ξ€Œξ€ξ€Žξ€Œξ€ξ€‹ξ€ξ€
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
β€’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

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The next closest star – called Barnard’s 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
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
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
oA star’s 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 star’s position in
the sky
Imagine – Two pictures of the sky around Barnard’s star are taken 22-years
apart from one another
oWhen the pictures are superimposed on one another:
It is clear that Barnard’s star has moved within this 22-year
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 Bernard’s star are not (they are in different
positions in either picture)
β€’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
Earth’s motion around the Sun
oInstead, it indicates real spatial motion of Barnard’s
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
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