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TitleHSC Physics Astrophysics Notes
TagsStars Apparent Magnitude Electromagnetic Spectrum Absorption Spectroscopy Angular Resolution
File Size804.9 KB
Total Pages26
Document Text Contents
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Parallax Angle (p)

HALF the corresponding angle
subtending at a star when the star is
viewed on two different paths from
earth (6 months apart.) The angle is
measured from the perpendicular to
the sight path:

*logically, the smaller the parallax
angle, the further away the star (pc)

Nearby Star A star less than 300 light years away.

Parsec (pc)

The distance that a star would have to be placed away from
earth in order subtend a parallax angle of 1”when lines are
drawn from either side of a 1AU base line (e.g. avg. radius
of earth’s orbit around the sun)

1pc = 3.26 ly

Light-Year (ly)
The DISTANCE travelled by light in one year

1 ly = 9.5x10


Astronomical Unit

Average distance between the sun & earth (orbital radius)

1 AU = 1.496x10

1AU = 1.496x10


63 000 AU = 1 ly

Limitations of


 Distant stars have an angle too small to be measured
(smallest measurable angle is approx. p=0.01”)

 The Atmosphere blurs images, making angle harder to
measure (only stars <100pc away can be measured)

 Earth’s orbit is not completely circular (elliptical)

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The CNO reaction is much faster than the p-p reaction, causing the hotter, larger blue stars

to expire faster. This is also why the CNO reaction releases heat faster.

The core Helium content reaches 15%, the star transitions to a red giant or supergiant and

begins undergoing Triple-alpha reactions to fuse Helium into heavier elements (such as

carbon and iron.) Three helium atoms combine to form Carbon, and Carbon and Helium

atoms combine to form oxygen:

He2 

C6 + gamma + energy

Star Clusters

Depending on their age, star clusters may be either open star clusters or globular clusters:

 Open Star Clusters are NEWCLUSTERS – they have no red giants or white dwarfs in

them. The stars of open clusters are just babies – all main sequence stars. (e.g. Pleiades)

 Globular Clusters are OLD CLUSTERS - typically globe shaped with millions of distant

suns. Many older stars – red giants and white dwarfs – are present, because hotter,

bluer stars age faster than the cooler main sequence stars. (e.g. Omega Centauri) –

Visually, a globular cluster appears as a ‘blot’ of light at the centre, that disperses as it

moves out.

Hotter, larger stars age quicker – they use up their fuel faster and progress through their life

stages quicker.

Determining the Age of a Star using H-R Diagrams

The age of a star can be determined by examining the cluster of which it is part:

 All the stars in a cluster are about the same age and distance

 Open Clusters are young, sparse clusters consisting of a few hundred loosely bound

stars. Open cluster stars have spectra that reveal and abundance of metals.

 Globular clusters contain hundreds of thousands of stars bound together in a rough

sphere. Globular clusters have a fewer metallic spectral lines.

 The metal abundance of a cluster is an indicator of its age.

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Open Cluster Globular Cluster
MB type stars occupy the main sequence
(most of the stars are main sequence)

Only the lower portion of the main sequence
is present (cooler stars age slower)

No Red Giants / White Dwarfs Many red giants / White dwarfs / highly
luminous, large stars

Stellar masses > 0.1M All stellar masses <0.8M

 In open clusters, young, hot stars occupy the main sequence.

 Larger stars have a shorter ‘incubation’ period – take less time to form

 Stars greater than 8 solar masses go onto form other heavier elements until Fe is formed

Star Death

A star dies when fusion reactions in the core cease and the outward pressure of radiation is

insufficient to balance the compression of gravity.

Stars of comparable size to our sun (1-5 solar masses) undergo the following death:

 Once the red giant stage has ended, the star unleashes numerous ‘bursts’ of luminosity

as it disperses layers of its atmosphere. These gases ejected from the star appear as

‘rings’ known as planetary nebula, from which new stars form.

 The core collapses into a white dwarf – heat and energy condensed into a small, dense

mass that still possesses kinetic energy to keep it spinning. It cools very slowly due to its

small surface area.

Massive stars (5-8 solar masses) undergo the following death:

 Supergiant star has fused lots of lighter elements into heavier ones, the heaviest being

Iron. The core collapses catastrophically, increasing the brightness of the star

dramatically in its supernova stage.

 A neutron star forms if the star was 1.4 solar masses +

 More massive stars form black holes

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