Download File - YEAR 11 EBSS PHYSICS DETAILED STUDIES

Unit 1 Physics
Detailed Study 3.2
Chapter 11: Astrophysics
Detailed Study 3.2
11 Astrophysics
 Key Knowledge and Skills
Section 11.1
The stars – how far, how bright?
Astrophysics
What is the universe?
How and why did the conditions for life to evolve
occur?
Galileo realised that the Earth circled the Sun
By Newton’s time it was realised that the stars must be
other suns
Newton calculate that the stars must be millions of
times further away than our Sun.
Section 11.1
The stars – how far, how bright?
Far far away
 The distance to the stars can be measured by the
parallax movement that they show as a result of the
Earth’s revolution around the sun.
 Even the largest parallax found is less than 1 arcsec.
(James Bradley, 1729)
 Telescope technology became
an important factor
 William Struve, 1835 - Vega
 Some stars are out of range.
Section 11.1
The stars – how far, how bright?
Far far away
Section 11.1
The stars – how far, how bright?
Measurements:
Arcmin – arc minutes
 Degrees are broken up into 60 minutes
Arcsec – Arc seconds
 Arc minutes are further divided by 60 arc seconds
 1 arc second is 1/3600 of a degree (60*60=3600)
AU – radius of the Earths orbit around the Sun
Parsec – (pc) - parallax angle
 caused by the radius of the Earth to the distance of the star
 1 parsec is the distance to a star that would show 1 arcsec of
parallax. (206 265 AU)
Light-year – (l.y.)
 Distance that light travels in 1 year
Section 11.1
The stars – how far, how bright?
Measurements
Section 11.1
The stars – how far, how bright?
Starlight – how bright?
 Astronomers measure the apparent brightness of stars
similar to an ancient scale created by Hipparchus 2nd Century
BC.
 First-magnitude (+1) stars were the brightest stars he could
see, second-magnitude (+2) was around half as bright, and so
on all the way to sixth-magnitude (+6) which were barely
visible to the naked eye.
 This scale worked well… until astronomers sailed south and
discovered stars brighter then first-magnitude… slightly
problematic.
Section 11.1
The stars – how far, how bright?
Starlight – how bright?
The discovery of stars brighter then first-magnitude
extended the apparent magnitude scale upwards to 0 and
then -1 and so on.
The invention and development of telescopes allowed
for the discovery of stars dimmer then +6, so the scale
was extended downwards, +7 and so on.
In the 19th century astronomers were able to more
accurately quantify the apparent magnitude of a star.
It was determined that each level of magnitude
represents a change in brightness of 2.5 times rather than
Hipparchus double.
Section 11.1
The stars – how far, how bright?
 Starlight – how bright?
Section 11.1
The stars – how far, how bright?
Brightness and luminosity
The actual brightness, or Intrinsic brightness measures
the total radiated power of a star, this is measured in
Watts and is know as Luminosity (L).
The apparent brightness (b) of a star can be determined
by calculating the amount of received radiation, this is
measured in Watts per square meter.
Luminosity L=b × 4πR2
Where L is the luminosity in Watts
b is the apparent brightness in W m-2
R is the distance to the star in m
Section 11.1
The stars – how far, how bright?
Stars come in many colours…
When you look up the night sky, you probably just see a
bunch of white dots. However a closer look (like with a timelapse using an SLR) will reveal a variety of different coloured
stars.
Just as the colour of a flame tells you how hot it is, the colour
of a star allows astrophysicists to determine the surface
temperature of a star.
Section 11.1
The stars – how far, how bright?
Stars come in many colours…
By looking at the apparent brightness of three different
spectra, Ultraviolet (U), Violet-Blue (B) and Visible (V) we can
determine the surface temperature of a star.
This can be done by taking a ratio of the brightness in V (bV)
to the brightness in B (bB).
A hot star will radiate mostly in the B spectra compared to
the V, so we would expect to have a small bV/bB ratio.