Unit 1 Physics Detailed Study 3.2 Chapter 11: Astrophysics

 Key Knowledge and Skills Detailed Study Astrophysics

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, 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. ( 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 2 nd 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 19 th 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 × 4πR 2  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 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 11.1 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 (b V ) to the brightness in B (b B ).  A hot star will radiate mostly in the B spectra compared to the V, so we would expect to have a small b V / b B ratio. Section 11.1 The stars – how far, how bright?