Lecture 36

Telescopes (continued).

Basic Properties of Stars.

• Observatories and Spacecrafts

• Stellar Brightness, Distances, Luminosities

Chapter 17.8 17.16

Refractors

Refractors

Reflectors

Reflectors

Types of Telescopes

Optical and Infrared telescopes

Radio telescopes (use metal “mirrors”)

Interferometeres (link several separate telescopes together to improve angular resolution)

Observatories

Radiotelescopes

Satellites

• The first satellite 1957 Soviet Sputnik

• First astronomical satellites late 1960’s

• The Hubble Space Telescope (HST) 1990

• The X-ray Chandra Observatory 1999

• The Spitzer Space (IR) Observatory 2003

Satellites

Important Stellar Parameters

Stars have similar internal structures and energy sources.

The most important parameter, which causes differences in a star’s appearance, is its mass.

The mass determines the star’s lifetime, surface temperature, radius, and luminosity at any moment.

Astronomers classify stars according their luminosities and surface temperatures.

Stellar Luminosity

Luminosity is the total amount of power the star radiates into space.It is measured in power units (Watts).

Brightness of a star in the sky depends on the distance towards a star and its luminosity.

The apparent brightness is the amount of light reaching us per unit area.

Apparent Brightness

Apparent brightness obeys an inverse square law with distance.

At the distance of Jupiter is 5 A.U., the Sun is 25 times dimmer than on Earth.

Alpha Centauri radiates almost the same amount of light as the Sun, but it is located 27,000 times further away from Earth than the Sun.Thus, its apparent brightness is 70 billion times less than that of the Sun.

The Inverse Square Law for Light

Luminosity – Distance Relation

LuminosityApparent brightness = ------------------- 4 π (distance)2

The units of apparent brightness are Watts per square meter.

Luminosity is also measured in the units of solar luminosity (LSun = 3.8 1026 Watts).

Measuring the Apparent Brightness

Stars emit radiation of all wavelengths.No detector is sensitive to the entire spectrum.

Usually we measure apparent brightness in a small range of the complete spectrum.

Eyes are sensitive to visible light.When we measure the apparent brightness in the visible region, we can calculate only the visiblelight luminosity.

Stellar Parallax

Parallax is the annual shift in a star’s apparent position in the sky due to the Earth’s orbital motion.

The parallax angle is half the annual shift.

The parallax angle of the nearest star, Proxima Centauri, is 0.77 arcseconds.

Parsec

An object with a parallax of 1 arcsecond is located at the distance of 1 parsec.

1 pc = 3.26 light-years = 3.09 1013 km

1d (in parsecs) = -------------------------- p (in arcseconds)

Stellar Magnitudes

Historically stellar brightness is described in magnitudes suggested by Hipparchus.

The brightest stars received the designation ‘’first magnitude’’, the next brightest ‘’second magnitude’’, etc.

The faintest stars visible by the naked eye are ‘’sixth magnitude’’.

Stellar Magnitudes

The modern magnitudes system is more precisely defined. Since a star may have any brightness, fractional apparent magnitudes are possible.For example, a star of magnitude 1.00 is 2.5 times brighter than a star of magnitude 2.00.

The brightest star in the sky is Sirius with an apparent brightness of –1.46.The faintest stars observed with HST are of ~ 30th magnitudes.

Surface Temperature

Surface temperature determines a star’ color.

The coolest stars are red, the hottest ones are blue.

Only the brightest star colors can be recognized by the naked eye.The color can be determined better by comparing a star’s brightness in different filters.

Betelgeuse has a temperature of ~3,400 K,Sirius ~9,400 K, the hottest stars – up to 100,000 K.

Spectral Type

The surface temperature also determines the line spectrum of a star.

Hot stars display lines of highly ionized elements, while cool stars show molecular lines.

Stars are classified by assigning a spectral type.The hottest stars are called spectral type O, followed by B, A, F, G, K, M as the surface temperature declines.

Oh Be A Fine Girl, Kiss Me

Stellar Masses

It is harder to measure stellar masses.The best method is to apply Kepler’s third law in combination with Newton’s law of gravity.

This procedure can only be applied to orbiting objects:Visual binary – a resolved pair of stars (Mizar)Eclipsing binary – a pair orbiting in the plane of our line of sightSpectroscopic binary – an object with regularly moving spectral lines or with 2 line systems.

The Hertzsprung-Russell Diagram

Invented by Ejnar Hertzsprung (Denmark) and Henry Norris Russell (USA) in 1912.

The diagram is a plot of stellar luminosities against their surface temperatures.

Temperature increases leftward.Luminosity increases upward.

H-R diagram

Patterns in the H-R diagram

Main sequence – location of the most stars(from upper left to lower right corner)Luminosity class V

Supergiant branch – along the top (class I)Giant branch – just below the supergiants (class III)

White dwarfs – near left corner (small size, high temperature)

Star Clusters

Open clusters and globular clusters.

Open clusters contain a few thousands stars and span ~30 light-years (10 pc). Pleiades

Globular clusters can contain more than a million stars and span 60-150 light-years.

Stars in clusters are at the same distance from the Sun and are formed at about the same time.It is easy to determine clusters’ ages.

Star Clusters

Age of cluster = lifetime of stars at main-sequence turnoff point.

Most open clusters are relatively young (<5 billion years).Globular clusters are typically old objects (12-16 billion years), the oldest objects in the galaxy.They place a limit on the possible age of the Universe.

Summary

The differences between stars are due to their initial mass and current age.

The HR diagram is one of the most powerful tools of astronomers.

Stars spend most of their lives at the main sequence. The most massive stars live a few million years, while the least massive stars live more than the current Universe age.