Unit 1 PhysicsDetailed Study 3.2Chapter 11: Astrophysics

Key Knowledge and Skills

Detailed Study 3.211 Astrophysics

AstrophysicsWhat is the universe?How and why did the conditions for life to evolve occur?Galileo realised that the Earth circled the SunBy Newton’s time it was realised that the stars must be other sunsNewton calculate that the stars must be millions of times further away than our Sun.

Section 11.1The 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 becamean important factor

William Struve, 1835 - Vega Some stars are out of range.

Section 11.1The stars – how far, how bright?

Far far away

Section 11.1The stars – how far, how bright?

Measurements:Arcmin – arc minutes

Degrees are broken up into 60 minutesArcsec – 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 SunParsec – (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 11.1The stars – how far, how bright?

Measurements

Section 11.1The 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 11.1The 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 11.1The stars – how far, how

bright?

Starlight – how bright?

Section 11.1The stars – how far, how

bright?

Brightness and luminosityThe 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 11.1The 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 time-lapse 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 11.1The 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.

Section 11.1The stars – how far, how

bright?