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PHYS 1403 Stars and GalaxiesTopics for Today’s Class

1. Measurementsa) Measurements

b) SI units

c) Conversions

d) Scientific Notation

e) Uncertainty

2. Chapter 1: Hear and Nowa) Scale of Universe

b) Astronomical Unit

c) Light Year

3. Chapter 2: The Skya) Constellations

b) Magnitude and Brightness of Stars

Review from High School Math

MeasurementsA tool to use the laws of mathematics to solve problems and to be able to distinguish between logical and illogical arguments.

The most fundamental measurements are that of; Length L

Mass M

Time T

System of Units

System International (SI)A modern form of metric system

Conversion from MKS to FPS

http://www.tpub.com/doephysics/classicalphysics6.htm

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Scientific Notation

Astronomical numbers are very large or very small so Astronomers use Scientific Notation

Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu).

Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu). Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu).

Significant Numbers

Getstartedinsicence.weebly.com

Significant Numbers

slideplayer.com

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Significant Numbers Uncertainty

Scales of Size and Time

Astronomy deals with objects on a vast range of size scales and time scales.

Most of these size and time scales are way beyond our every-day experience.

Humans, the Earth, and even the solar system are tiny and unimportant on cosmic scales.

A Campus Scene

16 x 16 m

A City View

1 mile x 1 mile

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The Landscape of Pennsylvania

100 miles x 100 miles

The Earth

Diameter of the Earth: 12,756 km

Earth and Moon

Distance Earth – Moon: 384,000 km

Earth Orbiting Around the Sun

Distance Sun – Earth = 150,000,000 km

Earth Orbiting Around the Sun (2)

In order to avoid large numbers beyond our imagination, we introduce new units:

1 Astronomical Unit (AU) = Distance Sun – Earth = 150 million km

The Solar System

Approx. 100 AU

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(Almost) Empty Space Around Our Solar System

Approx. 10,000 AU

The Solar Neighborhood

Approx. 17 light years

The Solar Neighborhood (2)

Approx. 17 light years

New distance scale:

1 light year (ly) =

Distance traveled by light in 1 year

= 63,000 AU = 1013 km

= 10,000,000,000,000 km

(= 1 + 13 zeros)

= 10 trillion km

Nearest star to the Sun:

Proxima Centauri, at a distance of 4.2 light years

The Extended Solar Neighborhood

Approx. 1,700 light years

The Milky Way Galaxy

Diameter of the Milky Way: ~ 75,000 ly

The Local Group of Galaxies

Distance to the nearest large galaxies: several million light years

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The Universe on Very Large Scales

Clusters of galaxies are grouped into superclusters.

Superclusters form filaments and walls around voids.

What are Constellations?

• In ancient times– Constellations = brightest stars that appeared

to form groups• Represented great heroes and mythological

figures

– Position in the sky told stories handed down from generation to generation over thousands of years

Constellations – An Ancient Heritage

Constellations

Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright

stars in those regions.

Easily Recognizable Constellations and Their Brightest Stars

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Constellations of the Zodiac Constellations• There are 88 Constellations

• 12 of these hold special significance because the Sun passes through them in the course of a year. They are called Zodiacal constellations and are also used by Astrologers.

Apparent Motion of The Celestial Sphere

Apparent Motion of The Celestial Sphere

Star Names in a ConstellationStars are named by a Greek letter () according to their relative brightness within a given constellation + the possessive form of the name of the constellation:

OrionBetelgeuse

Rigel

Betelgeuse = OrionisRigel = Orionis

Order of Greek Letters

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Consider a Real Nigh Sky PhotographIt is clear that not all stars are the same brightness.

How do you measure the brightness of a Star?

www.startribune.com

Measuring the Brightness of Stars

• Greek astronomer Hipparchus (160-127 BC) invented a number system to measure brightness of stars based on their appearance of size

– Brightest stars: ~1st magnitude

– Faintest stars (unaided eye): 6th magnitude

Gcseastronomy.co.uk

This scale is subjective and does not have a quantitative basis

Modern Definition

• In1856 Norman Pogson proposed that the eye perception of light is logarithmic so five magnitude difference corresponds to 100 2.512 , consequently 1st magnitude star is 2.5 times brighter than 2nd magnitude star and the 3rd

magnitude star is 2.5 x 2.5 = 6.25 times brighter than 1st magnitude star.

Gcseastronomy.co.uk

Larger the magnitude number, fainter the brightness of star

Intensity (Flux) and Magnitude Difference

• This table is one way to remember therelationship between brightness andmagnitude.

Equation wise• Apparent Magnitude (mv): Brightness of the star

irrespective of its distance from us• Apparent magnitude versus intensity (flux)

m = apparent magnitudeI = intensityA = Star AB = Star B

• Intensity versus apparent magnitude

2.5 BA B

A

Im m Log

I

(2.512) B Am mA

B

I

I

Examples

• Two stars differ by 3 magnitude. What is the intensity ratio?

• Sirius is 24.2 time more intense than Polaris. What is the magnitude difference?

2.5 24.2 2.5 1.38 3.5A Bm m Log

3(2.512) 16A

B

I

I

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The Modern Magnitude Scale

Sirius (brightest star in the sky): mv = -1.42Full moon: mv = -12.5

Sun: mv = -26.5

The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects):

Example

Acknowledgment

• The slides in this lecture is for Tarleton: PHYS1411/PHYS1403 class use only

• Images and text material have been borrowed from various sources with appropriate citations in the slides, including PowerPoint slides from Seeds/Backman text that has been adopted for class.