C 13 Stars and Galaxies Section 1 Stars

C 13 : Stars and Galaxies

Section 1 : Stars • • • Demo : expanding universe (p. 369) Q: What is a star ? A: large, massive, hot ball of gas held together by gravity and gives off light. Q: What is a Constellation? A : a number of stars that appear to form a pattern.

Constellations

• Constellations are not real • The purpose is to help to locate stars.

• Stars in a constellation often have no relationship to each other. • Modern astronomy divides the sky into 88 constellations.

Circumpolar Constellations • As Earth rotates, Ursa Major & Ursa Minor and other constellations in the northern sky circle around Polaris. • They appear to move because Earth is rotating. • The stars appear to complete one full circle about 24 hours. • Visible all year long due to their unique position.

• As Earth orbits the Sun, different constellations come into view while others disappear.

Absolute & Apparent Magnitudes • When you refer to the brightness of a star, you can refer to its absolute magnitude or its apparent magnitude. • Absolute magnitude – actual brightness of a star • Apparent magnitude – brightness of a star that we see

Parallax • It can be used to determine the distance of the star from the Earth. • Parallax is the apparent shift in the position of an object when viewed from 2 different positions. • Demo • The nearer an object is to the observer, the greater its parallax.

• Good to cal. distance for near star. • The parallax angle of the closer object is greater than that of the farther object.

Properties of Stars • Color: depends on temperature from coolest to hottest : red, orange, yellow, white, blue • Size : depends on original mass of gas cloud and age/lifestage of star from smallest to biggest : white dwarf, mainsequence, giants, supergiants

• Brightness : depends on surface temp, size, dist from Earth • The light from a star comes from nuclear reactions in center of a star • A star must have a balance between : 1. press due to temp in core 2. Gravity • Study the composition of stars by observing their spectra.

Section 2 : The Sun

• • It is the closest star to Earth. Center of the solar system. It is an enormous ball of gas. It is yellow in color. It is in the main sequence. Brightness : medium The size of the core ~ the size of Jupiter. 92 % hydrogen.

• The Sun has enough hydrogen to last 5 billion years. • It produces energy by nuclear fusion • Energy is transferred by convection and radiation.

The Sun’s Atmosphere • Photosphere – lowest layer of the Sun’s atm. The layer from which light is given off. 6000 K ~ 5700 ºC. • Chromosphere – the layer on top of photosphere. About 2000 km thick.

• Transition Zone – 2000 km to 10 000 km thick. • Corona – largest layer of the Sun’s atm. - extends millions of km into space. - 2 million K - charged ptles escape here as solar wind.

Surface Features • Sunspots – dark color spots on the surface becoz they are cooler than surrounding areas. - they are caused by intense magnetic storms on the Sun. - sunspots move because Sun rotates. Faster at equator (25 days/rotation) and slower at poles (35 days/rotation).

- sunspots are not permanent. - sunspot max : times when many large sunspots occur. Every 10 to 11 years. - sunspot min : occur in between sunspot max.

Prominences and Flares

• Prominence : the intensive magnetic fields associated with sunspots causes prominences. • They are huge, arching columns of gas.

• Solar flares : gases near a sunspot shooting outward at high speed. • CMEs (Coronal mass ejections) - occur when large amts of electricallycharged gas are ejected suddenly from corona. - 2 or 3 times each day during a sunspot max.

• It can damage satellites in orbits. • Interfere radio equipment. • It causes auroras. - high energy ptles contained in CMEs and solar wind are carried past Earth’s magnetic field → generates electric current toward Earth’s poles → ionized gases in atm →ions recombine with electrons →produce ight →aurora.

Aurora

The Sun – An Average Star • • • Middle-aged Brightness is average Yellow light Light reaches Earth ~ 8 mins Not close to any other stars Closest star system to the Sun – The Alpha Centauri System (triple star system)

Section 3 : Evolution of Stars • (I) Classifying Stars • High temps →brighter absolute magnitude • Hertzsprung – Russell (H-R) diag.

• • Most stars (abt 90% of all stars) fit into a diagonal band → main sequence Bigger, hot, blue, bright stars → upper left Smaller, cool, red, dim stars → lower right (most stars found here) Remaining 10% stars not found in main sequence :

1. White dwarfs – hot but not bright - small - lower left of H-F diag 2. Red Giants – large - bright but not hot - upper right - supergiants (largest giants)

Antares • • • Supergiant 400 light years from Earth 3 500 K 16 th brightest star in the sky 300 x diameter of Sun 11 000 times brighter than Sun

How do stars shine/generate energy? • Core of Sun → high temp → H atoms fuse into He → releases huge amt of energy • 4 H nuclei combine to create 1 He nucleus • Mass of 1 He nucleus < 4 H nuclei → mass is lost → energy is produced • E = mc 2 Eg: m=1 kg E = 1 x (3 x 108)2 = 9 x 1016 J → 2. 8 million US hours / year

Demo • P. 382 “make a model”

Evolution of Stars • When hydrogen nuclei (protons) depleted → star loses its main sequence status • Brightest stars → less than 1 million years to deplete • Dimmest stars → many billions of years • Sun – main sequence - life span ~ 10 billion years - 5 billion years left

• What is a Nebula? large and massive cloud of gas and dust particles

How a star is formed? • Due to supernova or collisions of 2 clouds of nebula → ptles of gas and dust exert gravitational force on each other → nebula contracts → instability within nebula → nebula breaks apart into smaller pieces → ptles in smaller pieces of nebula move closer → T increases (ptles collide more as they are closer) → protostar → core reaches 10 million K → fusion begins → star is formed and is stable → stars in main sequence

Star is stable in main sequence becoz heat from fusion causes Press increases → Press balances the gravity → when hydrogen in the core is depleted → gravity > press → core contracts → T inc in core → outer layer expands (more He) → outerlayer temp dec → core temp reaches 100 million K → He nuclei fuse to form Carbon → giant

How a white dwarf is formed? • After the star’s core uses much of its helium (this stage is after giant. The hydrogen fuel has used up → does not produce any new helium nuclei → helium combine to form carbon) → contracts even more (no more fusion → gravity pulls in) → outer layers escape into space → leaves behind the hot, dense core.

• Size of Earth • Eventually the white dwarf will cool and stop giving light

Supergiants and Supernovas • Stars more than 8 x massive than Sun → evolution occurs more quickly and more violently • Massive stars → core heats up to higher temps → heavier elements form by fusion (becoz higher temp is needed to fuse bigger elements. Eg : He → C needs higher temp) → star expands into supergiant → iron forms in the core → iron atomic structure does not release energy through fusion → core collapses (exploded) → shock wave

Neutron Stars • If the collapsed core of a supernova (after the explosion) is ~ 1. 4 → 3 times the mass of Sun → shrink to ~ 20 km in diameter → only neutrons can exist in the dense core → neutron star

Black Holes • If the collapsed core of a supernova is > 3 times the mass of Sun → continue to collapse → until it becomes a point → gravity near this mass is so strong that nothing can escape from it, not even light → black hole • Black hole is not like a giant vacuum cleaner sucking in distance objects • Only if the object crosses it → pulled into the hole • Stars and planets can orbit around a black hole

Section 4 : Galaxies and the Universe • Galaxy : a large group of stars, gas, and dust held together by gravity. (also includes planets) • Milky Way Galaxy : our galaxy • Galaxies are separated by huge distances – millions of light years apart. • Galaxies are grouped together into clusters • Stars (gp) → galaxies(gp) → clusters • Milky Way belongs to Local Group Cluster (~45 galaxies)

3 Major types of Galaxies 1. Spiral Galaxies • Whirlpool – like • Lots of dust in spirals • Closer to center →faster it orbits • Milky Way

2. Elliptical Galaxies • 3 -D ellipses : football • Round : soccer ball • No spirals • Very little dust

3. Irregular Galaxies • No recognizable shape • Smallest type of galaxy • 2 irregular galaxies orbit the Milky Way → Clouds of Magellan

The Milky Way Galaxy • ~ 1 trillion stars • Stars (including Sun) orbit around the core. It takes 225 million years for the Sun to make 1 round around the core. • Has a supermassive black hole at its center. It is about 2. 5 million times as massive as the Sun.

Origin of the Universe 1. Steady State Theory – the universe always has been the same as it is now. The universe always existed and always will. • Evidence indicates that the universe was different in the past.

2. Oscillating Model – universe began with expansion → expansion slowed → universe contracted →process began →oscillating back and forth. 3. Big Band Theory – started with a big band →expanding ever since.

Doppler Shift • If a star is moving toward Earth, its wavelength of light are compressed →light from the star is shifted to the blue end of the spectrum. • If a star is moving away from Earth, its wavelength stretched →light from the star is shifted to the red end of the spectrum.

• Red Shift → Universe is expanding • Quick Demo (p. 389)