View
213
Download
0
Category
Preview:
Citation preview
Physics Department 1/21/2018
Phys1411 Goderya 1
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
Physics Department 1/21/2018
Phys1411 Goderya 2
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
Physics Department 1/21/2018
Phys1411 Goderya 3
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
Physics Department 1/21/2018
Phys1411 Goderya 4
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
Physics Department 1/21/2018
Phys1411 Goderya 5
(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
Physics Department 1/21/2018
Phys1411 Goderya 6
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
Physics Department 1/21/2018
Phys1411 Goderya 7
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
Physics Department 1/21/2018
Phys1411 Goderya 8
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
Physics Department 1/21/2018
Phys1411 Goderya 9
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.
Recommended