In the direction of a particular star cluster, interstellar extinction allows only 15% of a star’s light to pass through each kiloparsec (1000 pc) of the interstellar medium. If the star cluster is 3.0 kiloparsecs away, what percentage of its photons survive the trip to the Earth?
126683
The percentage that gets through is 15% of 15% of 15% which is 0.3375%.
3953
Nothing in particular. There is a big 'boom' (with no sound because there is no air to vibrate) and there is a lot of light and a release of radiation, but that is about it. Basically, take an explosion on earth, and take away the earth, and voila.
259305
You are assuming there is an absolute reference to measure their speeds against. All speed has to be measured against some standard. But, for the sake of your discussion, let's assume you have two large asteroids in space that are motionless relative to each other. You launch super SciFi spaceships from each, aiming near to the other so they will pass. Each spaceship accelerates to 60% of C (speed of light). You are asking if they won't see the other zoom by at 1.2C.\n\nYou are forgetting that as they move to 60% of C, time is also affected. (google for the twins experiment). Time slows to an extent that they will never see anything approaching at C or in excess of C.\n\nHope this helps.\n\nChuck Taylor\nhttp://groups.yahoo.com/group/lunar-observing/
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Well, obviously the sun doesn't stop outputting light just because we're cloudy.  We're still getting 1120 W/m^2 through the atmosphere.  It's just being reflected, scattered, or absorbed by clouds, moisture, etc.\n\nSince clouds appear white from space, that says that a big chunk of the sunlight that would otherwise have it the ground winds up sailing off into space.
476063
It has to do with the light it recieves from the nearby stars but it also depends on what the gas cloud is made of.  Hydrogen emits (and blocks) red light.  Other forms of light are also reradiated as red.  As the temperature of the gas cloud increases (due to recieving radiation) infrared emissions would increase also.
550857
light is (in a nutshell) waves of energy. Waves (in general) travel in a straight line. stars are so far away that not much of their energy or waves hit the planet.
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I worry more about mass extinction events like an asteroid impact or finding out that giant aliens have just been raising humanity as a food source to harvest en masse. Bird flu might do a job on us or some alien organism brought back by space exploration which our bodies are unable to manufacture antibodies. Maybe global nuclear conflict will leave the earth a smoking glow-in-the-dark beacon rotating and wobbling in the solar system. Excuse me I have to go find my aluminum foil hat to block the gamma rays. Wait what if planet Jupiter is a brown dwarf star and decides to go nova? What if the sun flames out? What if the magma in the earth's crust makes the poles flip positions? .........
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Yes, it is true that there are stars and galaxies whose light has not yet reached Earth. This is because light travels at a finite speed (about 3.00E8 km/s). It does not lose energy to space either... and the only thing in space that disrupts light's path by any real amount is gravity.\n\nWhen we look into the sky at night we are seeing the past. When we look at our Sun we are looking 8 minutes into the past (light from the Sun takes about 8 minutes to reach us)... The nearest stars to our Solar System are hundreds, even thousands of light-years away (a light-year is the measure of the distance that light travels in a year)... So, when you look at the stars in the sky, you are seeing them as they were 100's, 1000', if not millions or billions of years ago. Scientists, using our most advanced telescopes have looked as far back as when our Universe was only a baby (about 450 million years old). As the Universe is about 15-20 billion years old right now thats very far back in time.\n\nTo answer your question about faster-than-light travel. As we know physics today, that is impossible. Einstein and many other relativistic theortists have shown time and time again that faster-than-light travel has a high probablity of impossibility. Also, light speed and (if possible) faster than light speed travel would affect the traveler's perception of time relative to the inertial frame.\n\nThis is a basic theory in relativity and I really don't want to get into that.
451667
It's a bit complicated.  The Earth's magnetic field is powerful enough to minimize charged particles from the solar wind, and the atmosphere attunates almost all other particles.  However, when a star goes nova, two jets are created at the poles.  These jets are extremely powerful with respect to the rest of the explosion.\n\nTherefore, if the Earth were not in the line of sight of the jets, the atmosphere and magnetic field would be extremely effective, and a supernova would have to be with a few 10s of light years to be dangerous.  However, if the Earth were in a line of sight of the jets, then during a few seconds the Earth could be bathed in deadly radiation, even if the source were a thousand light years away.
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one is the frictionless ship. being entirely frictionless is physically impossible, at least on earth. another would be the improbability drive. you cannot travel thru space on pure improbability. another scientific problen is the fact that almost every planet is inhabited by an alien species, and that most are humanoids. a formula developed by a scientist named frank drake. it is an equation for the probability of alien civilizations in this galaxy with intelligences like or above ours. N = N* fp ne fl fi fc fL. each variable is explained thus:The equation can really be looked at as a number of questions:\n\nN* represents the number of stars in the Milky Way Galaxy \nQuestion: How many stars are in the Milky Way Galaxy? \nAnswer: Current estimates are 100 billion. \nfp is the fraction of stars that have planets around them \nQuestion: What percentage of stars have planetary systems? \nAnswer: Current estimates range from 20% to 50%. \nne is the number of planets per star that are capable of sustaining life \nQuestion: For each star that does have a planetary system, how many planets are capable of sustaining life? \nAnswer: Current estimates range from 1 to 5. \nfl is the fraction of planets in ne where life evolves \nQuestion: On what percentage of the planets that are capable of sustaining life does life actually evolve? \nAnswer: Current estimates range from 100% (where life can evolve it will) down to close to 0%. \nfi is the fraction of fl where intelligent life evolves \nQuestion: On the planets where life does evolve, what percentage evolves intelligent life? \nAnswer: Estimates range from 100% (intelligence is such a survival advantage that it will certainly evolve) down to near 0%. \nfc is the fraction of fi that communicate \nQuestion: What percentage of intelligent races have the means and the desire to communicate? \nAnswer: 10% to 20% \nfL is fraction of the planet's life during which the communicating civilizations live \nQuestion: For each civilization that does communicate, for what fraction of the planet's life does the civilization survive? \nAnswer: This is the toughest of the questions. If we take Earth as an example, the expected lifetime of our Sun and the Earth is roughly 10 billion years. So far we've been communicating with radio waves for less than 100 years. How long will our civilization survive? Will we destroy ourselves in a few years like some predict or will we overcome our problems and survive for millennia? If we were destroyed tomorrow the answer to this question would be 1/100,000,000th. If we survive for 10,000 years the answer will be 1/1,000,000th. \nWhen all of these variables are multiplied together when come up with: \nN, the number of communicating civilizations in the galaxy. \n\n\n--------------------------------------------------------------------------------\n\n\nThe real value of the Drake Equation is not in the answer itself, but the questions that are prompted when attempting to come up with an answer. Obviously there is a tremendous amount of guess work involved when filling in the variables. As we learn more from astronomy, biology, and other sciences, we'll be able to better estimate the answers to the above questions. Many of these questions will be addressed in depth in future issues of Enigma.
232316
Ozone (O3) reflects/scatters ultraviolet radiation back into space.  However, if it is depeleted, this radiation makes it past the atmosphere and down to the surface.  The amount of shorter wavelength (higher energy) radiation passing through the atmosphere would increase, causing higher mortality rates from skin related ailments and can also destroy plant tissue.  Also, this radiation would heat the earth more since it carries more energy with it.  This is not usually a problem, since this thermal radiation currently passes back into space.  However, with pollution clouds/dust being more prominent, it would reflect the thermal radiation back down to earth's surface, driving the temperature up even more.
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The main reason is dust in space that absorbs the light.  For instance, we are in a great big spiral galaxy.  Generally, if you look at a spiral galaxy, you see an immense, bright core.  So why don't we see such a core?  It's because of the dust.  The core, if visible, should be right around the constellation of Sagitarius.  But since there's so much dust in the plane of the galaxy, we can't see it in the invisible spectrum.  Astronomers have managed to infer the direction of the center of the galaxy by looking for a type of light (21 cm radiation) that is regualrly produced by astronomical phenomena, and which has an ability to pass more easily through the clouds of dust in space.  By gauging the direction of most of this radiation, they can infer that the bulk of the galaxy is in the direction of Sagitarius.\n\nAnd really, there is "light" coming from all directions, but most of it isn't in the visible range.  Cosmologists have found what is refered to as the 3K (three "degrees" Kelvin--a very cold temperature) radiation background that exists in direction.  This is seen not as the product of stars, but as a remnant of the big bang.\n\nBut most importantly, there are not really "infinite" stars.  The universe, though boundless, is not truly infinite.  There are some mathematical oddities that you find when you try to deal with the infinite and the boundless.  For instance, there are an infinite number of poritive integers, and there are an infinite number of irrational numbers between 0 and 1.  As odd as it seems, there are actually more irrational numbers between 0 and 1 (what my math professor called the "uncountably infinite") than there are positive integers (so-called "countably infinite").  Due to the relativity of simultneity shown by Einstein, one cannot exactlt quantify the total number of stars in the universe at a given point in time, because an observer moving at a different speed and looking at that moment in time would disagree as to which stars had already exploded or not, and which stars had already formed or not.  So while you can really make a definitive statement as to the number of stars, the number is not really infinite in the general sense of the word, as neither is the universe truly infinite.
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each magnitude corresponds to a ratio of 10^(-.2), so you want to multiply the (-.2) by (1.9) and look at 10 to that power. This gives the  fraction of photons that make the trip.\n\nHomework and repeat question.
408726
Earth: The Lone Pale Blue Dot?\nNovember 02, 2006 \n\nA recent photo from the Cassini spacecraft shows the mighty planet Saturn, and if you look very closely between its wing-like rings, a faint pinprick of light. That tiny dot is Earth bustling with life as we know it. The image is the second ever taken of our world from deep space. The first, captured by the Voyager spacecraft in 1990, stunned many people, including the famous astronomer Carl Sagan who called our seemingly miniscule planet a "pale blue dot" and "the only home we've ever known." \n\nLast September, a portion of Cassini's picture showing Earth was unveiled to an auditorium full of scientists attending the third Pale Blue Dot workshop at the Adler Planetarium in Chicago. The workshop brought together scientists from across the country and beyond to talk about how to find life elsewhere in our universe, a central theme in the interdisciplinary field of astrobiology. When the workshop participants were presented with the picture, they spontaneously began to clap. One of their fundamental goals is to capture a portrait like Cassini's showing another "pale blue dot" like Earth in a planetary system beyond our own. \n\nIf scientists do ever acquire such a photo, how will they figure out whether anybody is home? Workshop participant Dr. Wes Traub of NASA's Jet Propulsion Laboratory in Pasadena says a photo of a light-blue extrasolar planet - a planet outside our solar system - might indicate the planet has an atmosphere, a tantalizing sign that it could support life. "In our solar system, we are the only small light-blue planet," he said. Earth appears blue because its atmosphere scatters blue light around, literally filling the sky with the color blue. \n\nBut colors are only clues. A planet with an atmosphere might be blue, or not. Mars has an atmosphere, albeit a thin one, and it's red. The blue light in the martian skies is soaked up by iron-containing molecules on the surface that radiate red. \n\nUltimately, astrobiologists will need additional data to determine whether an extrasolar planet is habitable, and if so, whether it harbors life. Future planet-finding missions like NASA's Terrestrial Planet Finder are expected to return both valuable "spectral" data and imagery of planets looking small enough to fit on a pin. Spectra are surveys of an object's rainbow-like array of different wavelengths of light. They reveal the presence of specific molecules. \n\nTo determine if a planet is livable, scientists will look for carbon dioxide and water vapor, signposts that a planet has an atmosphere and oceans, respectively. Atmospheres not only provide air to breathe, but also act like blankets to keep a planet warm and help buffer potential residents from damaging ultraviolet and cosmic rays. Oceans help regulate a planet's temperature and provide liquid water, an essential ingredient for life on Earth. \n\nOther molecules enveloping a planet, such as oxygen, ozone and methane, can suggest that life itself has taken root. On Earth, oxygen is "breathed out" by plants, and methane by micro-creatures living in swamps and animals. These chemicals don't stick around for long on their own, so if they are hanging around an extrasolar planet, then something must be pumping the stuff out. That something could be life, but this isn't always the case. Saturn's moon's Titan is shrouded in an atmosphere containing lots of methane not produced by life. \n\nScientists say that oxygen is a more reliable sign of life than methane, but if they found large quantities of both, they'd be more convinced. "Finding two of these molecules together would be much better than one. The more, the better," said Dr. Victoria Meadows of NASA's Spitzer Science Center, Pasadena, who served as chair of the third Pale Blue Dot conference. "For example, if we found carbon diox
363865
Once a scientist tried to calculate it by math, he said that only in our galaxy there are about 100 million suns, our sun has 9 planets to follow, while other suns could have more or less, so he said that each star could have 5 planets medium, this makes 500 million planets in our galaxy , medium , then he said that 1 of each 100 planet could be warm like ours , this makes 5 million planets , and one of each 100 of those , could have suitable gravity and atmosphere and so a sort of life , this makes 50 thousand planets , and one of each 100 of those has some forms of intelligent life, this makes it 500 planets only in our galaxy.\n\nbut the equation is much easier, our earth is like a grain of sand in its own solar system , why could it be the only form of intelligent life host in the whole universe ?\n\nI believe that some sorts of intelligent life on other planets are asking the same question of yours, and they could never know that we even exist.
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Lets look at the maths. If every star in this galaxy was a grain of sand ( approx. 100 billion ) you would have enough sand to fill an Olympic size swimming pool. how many habitable planets around each star ? Between 0 and 5 maybe. \n  There are approx 100 billion galaxies. Again if each star was a grain of sand  there would be more sand than on every beach on earth. The chances are there is life elsewhere and in all probability the seeds of life on earth came from somewhere else  as explained by the panspermia hypothesis.\n   And the alien abductions? ,,,we're just studying  8)
69324
It is Olbers' Paradox.\n\nWhy isn't the night sky as uniformly bright as the surface of the Sun?  If the Universe has infinitely many stars, then it should be.  After all, if you move the Sun twice as far away from us, we will intercept one quarter as many photons, but the Sun will subtend one quarter of the angular area.  So the areal intensity remains constant.  With infinitely many stars, every angular element of the sky should have a star, and the entire heavens should be as bright as the sun.  We should have the impression that we live in the center of a hollow black body whose temperature is about 6000 degrees Celsius.  This is Olbers' paradox.  It can be traced as far back as Kepler in 1610, and was rediscussed by Halley and Cheseaux in the eighteen century; but it was not popularized as a paradox until Olbers took up the issue in the nineteenth century.