How distant are stars?

Open Ended

Opening Question:

How long does the light from the Sun take to reach Earth?

Things to consider:

- How fast does light travel?

- How far away is the Sun from the Earth?

Distance from Earth to sun: 93.692 million mi = 1.5 x 10¹¹ m

​

Speed of light: ​

3 x 10⁸ m/s

Distance = Speed x Time (d = v * t)

​1.5 x 10¹¹ = 3 x 10⁸ * t

t = ​500 seconds

• Divide by 60 to get minutes -> 8.3 minutes​

How distant are stars?

Light seconds, light years, light centuries: How to measure extreme distances - Yuan-Sen Ting​https://www.youtube.com/watch?v=Op3AYaJc0Xw&list=WL&index=12

1) Trigonometric Parallax

2) Standard Candles​

• ​Cepheid Variable

• ​Type Ia Supernovae

Methods to Determine Distance

Fill in the Blank

Which method is used for very far away stars? How is it calculated?

​To determine stars that are farther away, we use the standard candle method.

• Light sources which are further away appear fainter because the light is spread out over a greater area.

• If we know how luminous a source really is, then we can estimate its distance from how bright it appears from Earth.​

Multiple Choice

Is a "light year" a speed ,

time, or distance?

• speed

• time

• distance

A light-year is the distance light travels in one Earth year.

Fill in the Blank

Which method is used for determining distance for objects closer by?

Trigonometric Parallax

• Used to determine the distance of nearby stars

• Apparent shift of object with respect to the background

Thumb Activity - Trigonometric Parallax

1) Stick out one thumb & close your left eye.

2) Open your left eye & close your right eye.

​​

• Looks like your thumb has moved while ​more distant objects have remained in place

Video

Introductory Astronomy: Parallax, the Parsec, and Distances

https://www.youtube.com/watch?v=XUQAIldqPww&list=WL&index=21

Fill in the Blank

What is an Astronomical Unit (AU)?

An Astronomical Unit is the distance from the center of the Sun to the center of the Earth .

• Approximately 93 million miles​

Multiple Choice

Is a "parsec" a speed ,

time, or distance?

• speed

• time

• distance

A parsec is a unit of distance used to describe distances between the nearby stars.

• Distance to a star that has a parallax angle of 1 arcsecond​

Multiple Choice

If the nearby star is close, then the parallax angle measured will be _______________.

• small

• medium

• large

• 0

Fill in the Blank

What is the closest star to the Sun?

The closest star to the Sun is Proxima Centauri, which is about 1.3 parsecs.

• Appears to shift by only the diameter of a dime at a distance of 2.4 km (about 4.2 light-years away)

• It is in the southern constellation of Centaurus, and is not visible from most of the United States.​

A bit about constellations

• ​Constellations are a group of stars that make an imaginary shape in the night sky.

• They are usually named after mythological characters, people, animals and objects.

• In different parts of the world, people have made up different shapes out of the same groups of bright stars.

Orion the Hunter

• For example, the constellation we now call Orion the Hunter was seen as different shapes by different people around the world, including:

  • China: Shen, the supreme warrior

  • Egypt: Orion or Osiris in Ancient Egyptian traditions

  • India/Hindus: Skanda, a celestial General riding a peacock​​

Constellations

• ​Today astronomers recognize 88 “official” Constellations. To be able to see all 88 Constellations, we would need to be at the equator.

• ​Brightest star in constellation is alpha, then beta, then gamma, then delta, etc

Review Video

Distances: Crash Course Astronomy #25

https://www.youtube.com/watch?v=CWMh61yutjU&list=WL&index=21​

Work on Study Guide until the end of class.

• Add information from today.