Astronomical Distance Scales

Some Common Distance Units:

• Light Year: the distance that light travels in one year (9.46 x 10^17 cm).
• Parsec (pc): 3.26 light years (or 3.086 x 10^18 cm).; also kiloparsec (kpc) = 1000 parsecs and megaparsec (Mpc) = 1,000,000 parsecs.
• Astronomical Unit (AU): the average separation of the earth and the sun (1.496 x 10^13 cm).

Some Representative Distances:

• The Solar System is about 80 Astronomical Units in diameter.
• The nearest star (other than the sun) is 4.3 light years away.
• Our Galaxy (the Milky Way) is about 100,000 light years in diameter.
• Diameter of local cluster of galaxies: about 1 Megaparsec.
• Distance to M87 in the Virgo cluster: 50 million light years.
• Distance to most distant object seen in the universe: about 13 billion light years (13 x 10^9 light years).

 

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Hubble's constant (Hubble's Law)

The Hubble constant H is one of the most important numbers in cosmology because it may be used to estimate the size and age of the Universe. Hubble constant indicates the rate at which the universe is expanding. Although the Hubble "constant" is not really constant because it changes with time (and therefore should probably more properly be called the "Hubble parameter"). The Hubble constant is often written with a subscript "0" to denote explicitly (clearly) that it is the value at the present time, but we shall not do so. 

The Hubble Expansion Law

In 1929, Edwin Hubble announced that almost all galaxies appeared to be moving away from us. This phenomenon was observed as a redshift of a galaxy's spectrum. This redshift appeared to have a larger displacement for faint, presumably further, galaxies. Hence, the farther a galaxy, the faster it is receding from Earth. The Hubble constant is given by
H = v/d where v is the galaxy's radial outward velocity, d is the galaxy's distance from earth, and H is the current value of the Hubble constant.

(Note -  In physics, redshift happens when light or other electromagnetic radiation from an object moving away from the observer is increased in wavelength, or shifted to the red end of the spectrum. In general, whether or not the radiation is within the visible spectrum, "redder" means an increase in wavelength – equivalent to a lower frequency and a lower photon energy, in accordance with, respectively, the wave and quantum theories of light. Redshifts are an example of the Doppler effect.)

Determining the Hubble Constant

Obtaining a true value for H is complicated. Two measurements are required. First, spectroscopic observations reveal the galaxy's redshift, indicating its radial velocity.

The second measurement, the most difficult value to determine, is the galaxy's precise distance from Earth. The value of H itself must be derived from a sample of galaxies that are far enough away that motions due to local gravitational influences are negligibly small (these are called peculiar motion, and they represent deviations from the Hubble Law).

Units for Hubble's Constant

The units of the Hubble constant are "kilometers per second per megaparsec." In other words, for each megaparsec of distance, the velocity of a distant object appears to increase by some value. For example, if the Hubble constant was determined to be 50 km/s/Mpc, a galaxy at 10 Mpc would have a redshift corresponding to a radial velocity of 500 km/s.

Current Value of the Hubble Constant

The value of the Hubble constant initially obtained by Hubble was around 500 km/s/Mpc, and has since been radically revised because initial assumptions about stars yielded underestimated distances. For the past three decades, there have been two major lines of investigation into the Hubble constant. One team, associated with Allan Sandage of the Carnegie Institutions, has derived a value for H around 50 km/s/Mpc. The other team, associated with Gerard DeVaucouleurs of the University of Texas, has obtained values that indicate H to be around 100 km/s/Mpc.

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What are the smallest particles in the Universe?

For many years, the only known subatomic particles were protons, neutrons and electrons. By the 1960s, however, advancements in particle accelerator technology had shown evidence of hundreds of smaller constituent particles. By studying these particles -- all part of what is known in particle physics theory as the standard model -- physicists can try to explain all of the forces and matter existing in the universe.

These smallest particles fall into several main categories, most notably fermions, hadrons and bosons.

Fermions

Fermions are the building-block particles. There are two types of material fermions: quarks, which work to hold the nucleus of an atom together, and leptons, which do not. Fermions can be broken down even further: There are different types of quarks, and for each, an anti-quark. Quarks are found in groupings, but leptons are found alone. Electrons and neutrinos are examples of leptons. Fermions have a half-integer spi.

Hadrons

Hadrons are composite particles made of smaller particles. A proton, for example, is a hadron made from a combination of different quarks. Strong interactions bind the hadrons together and they always have charges, but no color. Protons and neutrons are the most stable hadrons. Hadrons come in two classes: baryons and mesons.

Quarks

Quarks are the fundamental constituents of hadrons and interact via the strong interaction. Quarks are the only known carriers of fractional charge, but because they combine in groups of three (baryons) or in groups of two with anti-quarks (mesons), only integer charge is observed in nature.

Leptons

Leptons do not interact via the strong interaction. Their respective antiparticles are the anti-leptons which are identical except for the fact that they carry the opposite electric charge and lepton number. The antiparticle of the electron is the anti-electron, which is nearly always called positron for historical reasons. There are six leptons in total; the three charged leptons are called electron-like leptons, while the neutral leptons are called neutrinos. Neutrinos are known to oscillate.

Bosons

Bosons are subatomic particles that carry force. They help particles interact with one another without touching, much like the forces of gravity or magnets. Unlike fermions, bosons have integer spin. The Higgs boson is believed to be the tiny particle that likely provides mass to all matter. Yet scientists aren't even sure that the Higgs boson exists.The Higgs boson remains one of the key questions remaining in physics and in wrapping up the Big Bang theory. If scientists can identify and study the particle that gives mass to all others, they can explain how the universe started from a seemingly invisible field.

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Astronomy - August 2013 (40th Anniversary Issue)

Look forward to studying the starry night sky? Revel in seeing if you can locate and connect the Big Dipper and the brightest North star? If you're a star gazing enthusiast, you'll love Astronomy. You'll learn more about exploring the universe in your own backyard with the most popular amateur astronomy magazine.

• Astronomy - August 2013 (no ADS) 40th Anniversary Issue
• English | PDF | 83 pages | 36.6 MB

FEATURES :-

• 26 COVER STORY - 40 greatest astronomical discoveries
• Astronomers' biggest breakthroughs have lifted the veil on our universe. RICHARD TALCOT T
• 32 40 greatest mysteries of the universe
• Astronomers know more about the universe than ever but still have much to learn. SARAH SCOLES
• 38 Where will astronomy be in 40 years?
• The future involves larger collaborations, computers, and telescopes. DEBRA MELOY ELMEGREEN AND BRUCE G. ELMEGREEN
• 44 The Sky this Month
• Neptune's summer surge. MARTIN RATCLIFFE AND ALISTER LING
• 46 StarDome and Path of the Planets
• RICHARD TALCOTT; ILLUSTRATIONS BY ROEN KELLY
• 52 40 years of amateur astronomy
• We live in our hobby's golden age - just look at what's happened in the past four decades. MICHAEL E. BAKICH
• 58 Astronomy magazine's path to "stardom"
• From its modest beginnings, the publication now leads the astronomy hobby as the most
• popular magazine of its kind in the world. DAVID J. EICHER
• 68 Ask Astro
• Refracting light.
• 70 40 deep-sky targets in Sagittarius
• The Archer contains a dizzying variety of dazzling objects. MICHAEL E. BAKICH
• 72 Hunt down summer's best dark nebulae
• For a totally new observing experience, ignore the bright and aim for darkness. MICHAEL E. BAKICH
• 76 A backyard imager advances science
• An unexpected email opened the door for this astroimager. R. JAY GABANY
• 80 Prime time for Neptune and Uranus
• Late summer and early fall are the best times to track down the solar system's distant planets. RICHARD TALCOTT

COLUMNS :-

• Strange Universe BOB BERMAN 11
• Observing Basics GLENN CHAPLE 14
• Secret Sky STEPHEN JAMES O'MEARA 18
• Cosmic Imaging TONY HALLAS 24

QUANTUM GRAVITY :-

• Snapshot 9
• Breakthrough 10
• Astro News 12

IN EVERY ISSUE :-

• From the Editor 6
• Letters 11, 18,24
• New Products 84
• Web Talk 84
• Advertiser Index 87
• Reader Gallery 88
• Final Frontier 90

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Is the sun dying?

Without sun's existence, there wouldn't be life on Earth. In the grand scheme of things, however, our sun is simply another star among star among the other hundreds of billions of stars in the universe.Officially, the Sun is a class G2V star- in other words,a main-sequence yellow dwarf have a temperature range of 5,000 to 6,000 degrees Celsius and their mass is about 80-120 percent of the mass of the sun. That means that the Sun is one of the biggest yellow dwarfs in the group.

Like other yellow dwarfs, the Sun converts hydrogen to helium in its core through nuclear fusion,which generates massive amounts of energy and light.The Sun fuses about 620 million metric tons (683 million short tons) of hydrogen per sec. Based on the speed, astronomers believes that the sun is about halfway through its life cycle. About 40 percent of the hydrogen has been converted,leaving another 3 to 5 billion years before the Sun evolves into the next stage in its life cycle:a red giant.

 

But let's start at the beginning.The Sun formed approximately 4-5 billion years ago, at the same time as the rest of the solar system. At this time a spinning molecular cloud of dust, hydrogen and helium flattened out into a disk, With a gaseous sphere at its center that contained most of the mass.This sphere had a gravitational pull that attracted dust and other materials from the disk,which caused the sphere to compress until it begin converting the hydrogen to helium.A star-in this case, Our Sun-was born.

The Sun can't keep fusing hydrogen indefinitely, though - there's a finite supply. Nuclear fusion occurs in the Sun's core due to gravitational pressure, which heats the core to 15 million degrees (27 million degrees  Fahrenheit) and splits the hydrogen atoms.There's dedicates balance,known as 'hydrostatic' equilibrium 'between the inward compression exerted by gravity and the outward pressure from the energy created by nuclear reaction.As the Sun's hydrogen supply is used up,the nuclear fusion in its core will decrease, and the core will contract. The core will heat up to a temperature of 100 billion degrees Celsius and begin fusing helium into carbon.The outer layers of the Sun will expand as it become a red giant.This means that the Sun's radius will be 250 times larger than its current radius,and it will swallow the Earth.

       Astronomers once thought that because the Sun's gravitational pull will weaken when it becomes a red giant, the associated planetary movement outwards away from the Sun might spare our planet. However, the most recent projections show that Earth would likely still be in the outer layer of Sun,where it will be pulled in and vaporized.Even if the Earth itself is spared,astronomers still believe that the increasing heat from the Sun will eliminate life on Earth about one billion years from now.

Source - How it Works Book and Internet

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