Lesson 3.1: Measuring the Properties of Stars

Measuring the Properties of Stars

Measuring Distance with Parallax

• Stars move in the sky as the Earth moves around the sun

• Parallax: When an object shifts relative to the background. The closer it is, the more shift you see

•  The apparent change in any star’s position is very small, so small it isn’t measured in degrees but in arc seconds (1/3600 of a degree)

Multiple Choice

The constellation Orion was in the top left corner of the last slide. Which stars in the image are closer to Earth than the stars in Orion?

• Any of the stars that didn't move in the image

• Any of the stars that moved more than Orion in the image

• Any of the stars that moved a bit, but not as much as Orion did

Triangulation

• Using parallax, astronomers can measure the angle in arc-seconds and use it to calculate distance

• The distance to stars is measured in Parsecs, from the words parallax and arc-second

• One Parsec is 3.26 light-years (causing a shift of one arc-second)

Multiple Select

What does a Parsec measure?

• Time

• Distance

• Speed

Multiple Choice

What does a Light Year measure?

• Time

• Distance

• Speed

Measuring Temperature

The hotter a star is, the bluer it will be. Low energy stars will be much redder.

(Check out blackbody radiation if this concept interests you!)

Multiple Select

Select all of the following statements that are true:

• Bluer stars are hotter

• Bluer stars have more energy

• Redder stars are colder

• Redder stars have less energy

Measuring Luminosity

• Luminosity: The amount of energy radiated by an object (how bright it is)

•  A star’s luminosity measures how quickly it is consuming its fuel, which in turn is an important clue into the life span of the star. 

Multiple Select

Which of the following are true?

• The more luminous a star is, the darker it is

• The more luminous a star is, the faster it consumes fuel

• The more luminous a star it is, the more blue it is

• The more luminous a star is, the more energy it gives off

The Magnitude System

The Greek astronomer Hipparchus measured the apparent brightness of stars in the night sky back in 150 BC

He used units called magnitudes in which the brightest stars were magnitude 1 and the dimmest were magnitude 6

Astronomers still use this to measure brightness of astronomical objects today but they call it apparent magnitude to emphasize that they are measuring how bright it looks to an observer

Apparent magnitude depends on luminosity and distance

Multiple Choice

The higher the magnitude number, the brighter the star

• True

• False

Measuring Properties from Stellar Spectra

•  A star’s spectrum depicts the energy it emits at each wavelength 

• From the spectrum we can determine the star’s composition, temperature, luminosity, velocity in space, and rotation speed 

Spectral Types

•  After many years of work and different systems, stars have been classified in an order which arranges them by temperature, as indicated by their color

•  We now have O, B, A, F, G, K, and M stars, as well as a few others

•  To distinguish finer gradations in temperature astronomers also add a number 1-9 behind the letter, where smaller numbers equal higher temperatures

Multiple Select

Select all of the following that are true:

• An O1 star is the hottest type of star

• An A6 star is colder than an F1 star

• Most red stars are M class stars

• A G class star gives off more energy than a K class star

  L and T are classifications that have also been added to this system in recent years



Hertzsprung-Russell Diagrams

• If you plot stars by their luminosity and temperature, most of them lie in a nice diagonal line (called the Main Sequence)

•  Hot, luminous stars are in the upper left, and cool, dim stars are at the lower right

Multiple Choice

A H-R Diagram plots luminosity over temperature

• True

• False

Multiple Choice

Most stars are on a diagonal line through an H-R Diagram called the main sequence

• True

• False

• Most stars are found on the Main Sequence

• Stars in the upper right corner have the same temperature as those below them, but are more luminous which means they must be larger (red giants)

• Stars below the main sequence must be smaller if they are both hot and dim (white dwarfs)

Multiple Select

Select all of the following that are true:

• The stars above and to the right of the main sequence are Red Giants

• The stars below and to the left of the main sequence are White Dwarfs

• White Dwarfs are less luminous than the main sequence

• Red giants are hotter than white dwarfs

• White dwarfs give off more energy than red giants