Ask An Astronomer

did_stars_form_during_the_big_bang - created

czars — 2009-11-06T13:56:32-08:00

Hello, I have learned that the most distant galaxies accessible to us by telescope are about 14 billion light years away. Accordingly, the method used to determine the distance of said galaxies is to use the old light of supernovae that existed within those galaxies. These exploding stars mark the end of a stars long life, but the explosions themselves occurred some 14 billion years ago. My question is that if what I have explained is true, how were those stars formed before the big bang (13.7 billion years ago). Were they already formed in the hot condensed state of the singularity that has now come to be the universe?

Your question is a very insightful one. Normally if you want to know how long it takes for something to travel you can use the formula: velocity=distance/time or equivalently time=distance/velocity.

Using this formula on a distant galaxy you would arrive at the conclusion that you mentioned in your question.

However you have to keep in mind that the distant galaxy that emitted the light is not sitting still relative to us. In fact the space between us and that galaxy is expanding. As a picture to have in your mind: imagine that the universe is a loaf of raisin bread. Each raisin in the bread in this model represents a galaxy. Now imagine that you bake the bread and you put it in an oven. As the bread heats up, the yeast in the bread causes it to expand. Any two raisin in the bread are now moving away from each other in the bread.

Thus the reason why it is more distant than (the speed of light)x(age of the universe) is because it has been moving away from us since the light that we see was emitted.

I hope that answers your question. Feel free to shoot me back an email if you need more clarification.

Cheers,
Robert da Silva
UCSC

index

czars — 2009-11-06T13:54:33-08:00

Index

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blackholes

czars — 2009-11-06T13:53:49-08:00

Black Holes and Cosmology

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czars — 2009-11-06T13:53:33-08:00

UCO/Lick Ask An Astronomer Question Archive

This page collects all questions submitted to Ask An Astronomer and answered by one of the graduate students at the Astronomy Department, UCSC. Before submitting your question, please check if it has already been submitted by browsing the Archive.

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Send your questions to [askanastronomer@ucolick.org].

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Can we reveal the actual optical disk of another star than our sun's?

The answers are yes – yes, we have telescopes that large; yes, we've seen the disks of other stars, and yes, we're even directly imaging planets now.

First, as you've remarked, we have several large segmented-mirror telescopes that are on the order of 10 meters (~33 feet) in diameter. Interestingly, these were not the first telescopes to actually image a star. In '95-6, astronomers from Harvard used the Hubble Space Telescope (a diameter of only ~2.4 meters, ~8 feet) imaged Betelgeuse, a red supergiant in the constellation of Orion. (See the press release at http://www.cfa.harvard.edu/press/archive/betel97.html).

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_{Image Credit: Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA}

_{Disclaimer: The opinions expressed here are those of individual authors and don't necessarily represent the Observatory's positions. Send your feedback to askanastronomer@ucolick.org}

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if_the_universe_is_expanding_and_galaxies_are_rushing_away_from_one_another_why_are_the_galaxies_in_the_local_group_not_flying_apart - created

czars — 2009-02-02T13:24:43-08:00

If the Universe is expanding and galaxies are rushing away from one another, why are the galaxies in the Local Group not flying apart?

In the modern Universe, gravity is the main (often only!) force shaping the large-scale structure of the universe (galaxy-sized objects, groups of galaxies, and clusters of galaxies). The expansion of the universe wants to push galaxies further apart, but if enough mass is in a small enough area, gravity can stop the expansion of the universe in that region. This is why the planets are not moving away from the sun, why the galaxy doesn't fly apart, and why the Local Group of galaxies (the Milky Way, the Andromeda Galaxy, and several smaller galaxies) stays together.

Another question some people have is whether the “dark energy” of the universe, the “cosmological constant” that may be causing the expansion in the universe to accelerate, will result in our galaxy getting ripped apart in the future. Again, gravity wins, and everything up to our local group of galaxies will remain together in the future. There are larger structures in the Universe bound together by gravity, such as clusters of galaxies, where the local expansion of space has ceased. Superclusters of galaxies are known to exist, and these have enough gravitational pull to eventually overcome the expansion of the universe, but they haven't overcome the expansion yet.

The nearest cluster of galaxies is the Virgo cluster of galaxies. Currently, the Virgo cluster is receding from us at a rate of about 1200 kilometers per second. If there is no cosmological constant, the gravity of the Virgo cluster would stop this expansion, and the Local Group of galaxies would fall into the Virgo Cluster in the very distant future. But with the possible strength of the cosmological constant, it is no longer certain if the gravitational pull of the Virgo Cluster is strong enough to stop the Local Group of galaxies. If this is the case, then in the very distant future, the acceleration of the universe will push all the galaxies except for those in the Local Group beyond our sight, and we'll be left alone in a much duller Universe.

exactly_how_does_a_comet_s_tail_form_why_doesn_t_a_comet_s_gravity_hold_onto_the_gas - created

czars — 2009-02-02T13:23:58-08:00

Exactly how does a comet's tail form? Why doesn't a comet's gravity hold onto the gas?

The most important things acting on the comet and its tail are the gravitational field of the sun, the solar wind (which is a flow of fast-moving charged particles such as protons out of the sun), solar radiation (photons from the sun hitting the comet and particles in the comet's tail) and the gravitational field of the comet itself.

The gravitational field of the sun just causes the comet to orbit the sun. The solar wind and solar radiation “blow” the comet's tail so that it always points away from the star. Comets are commonly referred to as “dirty snowballs,” meaning that they are mostly ice with some dust and other “dirt” mixed in. The part of the comet's tail that we can see is mostly due to the sun's light reflecting off of dust in the comet's tail.

In principle there could be a stable “atmosphere” around a comet if the comet's gravitational field were strong enough to hold on to water and dust. However, comets are very small (typically a few kilometers in diameter), and it turns out that their gravitational field is just too small to hold on to any material that's not physically stuck to the comet.

To be a bit more quantitative, the escape velocity from the surface of a comet (how fast something needs to be moving to leave the comet and never return) is about 1 meter per second, around 2 miles per hour. So, if you were standing on the surface of a comet, you wouldn't find it too difficult to just pick up a snowball and throw it straight up so that it never came back. In fact, you could probably bend down and jump, and you'd exceed the escape velocity and therefore float off into space and never return to the surface of the comet. (Such a thing happened to an astronaut in the movie “Deep Impact.”)

Now, an object's temperature is one way of measuring the average speed of the particles that make up that object. Hot objects are made up of very energetic, fast moving particles, while cool objects are made up of sluggish, slowly moving particles. In order to make the average speed of water molecules faster than 1 meter per second, you have to have the gas warmer than about 0.001 degrees Kelvin, very close to absolute zero (this is about -459 degrees Fahrenheit). This is extremely cold, and a comet in the solar system will never get this cold, even when it's far from the sun.

The upshot of all this is that as soon as the sun heats up the comet enough to cause some of the ice to turn to water vapor, these water molecules are moving fast enough to completely overwhelm the gravitational field of the comet and escape. Then the dust and water vapor are blown back by the solar wind and solar radiation so that the comet's tail points away from the sun.

So no, there is no point where the comet's gravitational field balances the sun's radiation pressure. But this is only because comets are typically so small and therefore have weak gravitational fields. If comets were much more massive, there would be such a point. A good example of this is the Earth, with gravity strong enough to hold on to a fairly thick, warm atmosphere.

planets

czars — 2009-02-02T13:23:29-08:00

Planets, Asteroids and Comets

i_received_a_star_as_a_gift_or_purchased_it_for_a_friend_loved_one._can_you_confirm_that_it_exists_can_you_take_a_picture_of_it_for_me - created

czars — 2009-02-02T13:22:31-08:00

I received a star as a gift or purchased it for a friend/loved one. Can you confirm that it exists? Can you take a picture of it for me?

Many of the “Own-your-own or Name-your-own” star companies seem to report slightly inaccurate coordinates for their stars, though it shouldn't be a problem to obtain accurate coordinates. The stars “sold” are more-or-less randomly chosen from a list of catalogued stars, and the names/ownerships are not recognized officially or legally by any professional astronomical organization or government agency. (So don't try and collect royalties if the Hubble Telescope images “your” star!) Still, a lot of people enjoy the gift, and there is absolutely no harm in it.

You can find pictures of these stars at the online Digitized Sky Survey – pictures of the entire sky that have been scanned in to digital format. Go to the Digitized Sky Survey website and enter the coordinates. You may have to switch the “J2000” button to “B1950,” depending on which coordinates your star registry gave you. Be sure to change “File Format” from “FITS” to “GIF.” Chances are, your star is the brightest one in the field.

Lick Observatory does not take pictures of “your” star for people (unless you have several million dollars you'd like to donate), and it's appearance certainly has not changed since the sky survey images were taken. Re-taking the image would be a waste of the observatory's very limited resources.

Before “buying” a star, please read the International Astronomical Union's position on the naming of stars. Remember, you are buying a novelty gift. What you get (often a star chart with “your” star circled, a certificate with the star's name and coordinates, a spot in the company's records as owning a particular star, and perhaps a booklet with quasi-factual information about stars in general) is all that you are getting for your money.

what_kind_of_damage_do_solar_storms_cause_to_satellites - created

czars — 2009-02-02T13:19:52-08:00

I've read about the recent very large “solar storm” and would like to know what kinds of damage can be experienced by the many satellites in orbit around the earth. Are some types of satellite more likely to be damaged than others- ie communication vs optical like Hubble.

Solar storms can be energetic particles from the sun, disturbed regions of magnetic fields, large amounts of x-rays and radio waves from the sun, or combinations of these. Normally, Earth's magnetic field is strong enough that most satellites, even the weather satellites (which are in geosynchronous orbit 22,000 miles away) are within the magnetic field and therefore protected from most of the cosmic rays.

However, when a magnetic storm passes the Earth, Earth's magnetic controll shrinks. Often satellites in higher orbits (especially geosynchronous orbits) and orbits that cross Earth's poles can end up outside of Earth's magnetosphere, and these satellites are then exposed to damaging cosmic rays. Satellites in low orbits (like the space station, space shuttle, Hubble, etc.) are generally still protected.

The solar storm you heard about also featured a “proton event,” where high-energy protons from the sun stream toward the Earth. These protons have enough energy to penetrate Earth's magnetic field, and so all satellites are in danger from these storms. The worst danger is for spacewalking astronauts, who would receive very high (perhaps dangerously high) levels of radiation. Astronauts onboard the space shuttle and the space station are usually protected from these storms by special shielding.

You can read more about solar storms, and especially about currently active storms, from the Space Environment Center: http://www.sel.noaa.gov/

Thanks for writing!

Sincerely, Kurtis Williams

what_happens_to_dead_satellites_orbiting_the_earth - created

czars — 2009-02-02T13:19:09-08:00

What happens to all the satellites and other pieces of technology that are left in space? Do they continue to orbit our plant, disintegrate or are broken into small pieces, fall to or collide with other planets, or What? Is this a problem to be considered in this millennium?

The problem of space debris is a serious one. In the past, old satellites, pieces of rockets, and astronaut trash were left in Earth orbit. Things near the earth (within a few hundred miles) re-enter the atmosphere fairly quickly (within a few years) because there is still a little air to slow them down at these low orbits.

However, things in higher orbit remain in orbit forever. Anything with motors on them tend to explode eventually, creating swarms on tiny debris. Even dust from meteors can hit satellites, knocking off chips of paint. Even a fleck of paint can do serious damage at 17,000 miles per hour!

These days, satellite manufacturers are becoming more conscious about de-orbiting satellites when they are finished being used. The best examples are the Iridium satellites, a series of satellites belonging to a failed communications company. Rather than stranding the satellites in space, Iridium and their partners are spending several million dollars to de-orbit the satellites.

Problems still exist with existing trash, and various schemes have been developed for finding ways to clean up space. In the meantime, radar is used to locate larger pieces of space junk (sizes of nuts and bolts or larger). The space shuttle must routinely make orbital corrections to avoid such junk. The International Space Station recently announced that the shielding on the newest Russian module is too thin, and that the module has a 25% chance of being punctured by space debris during the life of the space station. So, space trash is still a serious concern!

Thanks for writing!

Sincerely, Kurtis Williams

what_are_some_major_challenges_of_human_space_exploration - created

czars — 2009-02-02T13:18:27-08:00

i'm in the 12th grade and the question is to research the environmental conditions on each of the planets in the solar system other than earth. and to evaluate each planet as a potential place for human colonization and to identify what challenges would be faced, and how they could be met?

Hello,

You've asked for much more information than I could type in a reasonable amount of time, so I hope you don't mind if I point you toward a good resource and give you some ideas to think about. The web site I am thinking of is a guide to the solar system by Students for the Exploration and Development of Space. Their web site is: http://www.seds.org/nineplanets/nineplanets/nineplanets.html This page has links to most objects in the solar system, and contains a lot of information about each one.

Here are some ideas to think about when reading each page and considering how humans could live there:

1) Air and Water - people need oxygen to breathe and water to drink and grow food. Taking water and oxygen from the Earth is very expensive and very difficult, so it would be easiest to colonize places where sources of water and /or oxygen are easy to find. (Note that if there is lots of water, oxygen can be made from the water. Also, if there is carbon dioxide, oxygen can be made.)

2) Food - people will need to grow their own food on another planet - transportation to and from Earth is, again, too expensive. Plants need carbon dioxide and sunlight (real sunlight is best; artificial lighting will work, too.)

3) Energy - human settlements will need to create their own energy. Close to the sun (say, within the asteroid belt), solar power is useable. Outside of the asteroid belt, nuclear power would most likely have to be used. There are advantages and disadvantages to each type of power.

4) Radiation - radiation is harmful for people, plants and animals. On Earth, we are protected by a magnetic field and an atmosphere. Some planets and moons have neither; some have only one or the other. Very few have both. Some planets have very strong magnetic fields that create even more radiation. Settlements will have to be protected to some degree.

5) Gravity - planets or moons with a lesser gravitational pull than the Earth will result in astronauts having smaller bone and muscle mass, a serious problem if they are to come back to Earth. The closer a planet's gravity is to Earth, the less of a problem this is.

6) Atmosphere - Some planets have no atmosphere. Some have atmospheres that will crush people. Generally, it's easier to deal with low-pressure rather than high-pressure, but both are dangerous if there is an air leak.

7) Distance - the further a planet is from Earth, the harder it is to get to and to talk to!

8) Temperature - Is the planet/moon too hot, too cold, or just right? The hotter or colder a planet is, the more energy it will take to change the temperature.

9) Miscellaneous - Some moons have active volcanoes that cover the entire moon with boiling lava regularly. Some planets have no ground. And there is psychology - how will people deal with living isolated on an alien rock for long periods of time? Etc.

There is a lot to think about, so you may want to focus in on certain of the above topics. All in all, Mars is probably the best choice for long-term human settlement. It may even be possible to change Mars' atmosphere to resemble the Earth! The moon is the most logical first step, because it is very close. Venus might seem like a good choice, but it is really a very bad choice. It is very hot, has a crushing atmosphere, and may be covered with molton lead.

There is another web site you might find useful: http://spaceflight.nasa.gov/mars/index.html This is a NASA web page that talks about exploring Mars and other planets.

Good luck, and thanks for writing!

Sincerely, Kurtis Williams

why_can_t_we_go_faster_than_the_speed_of_light_can_we_ever_colonize_other_planets - created

czars — 2009-02-02T13:17:51-08:00

1. Is the the speed of light the maximum speed and why/why not?

For a particle which is created travelling slower than the speed of light, the speed of light is the fastest possible speed. The reason is that, in order to accelerate an object, you need to use energy. The more massive (or “heavier”) an object is, then the more energy it takes to accelerate it. You understand this well, as it is easier to throw a ball at 50 kilometers per hour than it is to throw an automobile at 50 kilometers an hour. As an object moves faster, it appears to gain mass (or become heavier). We don't notice this in everyday life, because this effect really only becomes noticeable when you get very close to the speed of light, but scientists have measured and verified this theory in laboratory experiments. In fact, if an object gets very very close to the speed of light, it's mass goes up so fast that it can seem to be infinitely heavy! At this point, it is impossible to gain enough energy to accelerate the object any further.

2. E.g. if you travel in a ship that are well protected from paticle hits or are flying far from all material, would it be possible to gain lightspeed (theoretically) ? and could you increase speed from there ?

No, for the reasons I describe above. It would require infinite energy just to reach the speed of light, let alone accelerate beyond it!

3. If 2 photons emitts from the same source, are not their speed, compared to each other, twice the light speed ?

No. This is a very strange thing about special relativity. Einstein's theory states that, no matter whether you are moving or standing still, light always moves at the speed of light. So, if you were on one photon and looking at the other photon, it would appear to be moving at the speed of light. This is very complicated, and I really cannot explain it via email, but good books on special relativity should be able to help you somewhat. Mathematically, the ideas behind special relativity are very simple, and can be understood with basic algebra. However, a physical understanding of what special relativity means is very difficult to achieve. I'm still not certain that I truly understand it; I just accept it as true (since repeated experiments show these theories are true).

4. In your oppinion, how far are we from common use of spacetravel, collonization and space mining etc.? 50, 250 …?

For colonization and mining of objects in the solar system, we have the technology now to be able to accomplish these things. The major hurdles are economics (space travel is very expensive, and it is hard to justify sending people to an asteroid to mine iron when it is cheaper and safer to mine iron on the Earth) and physiological (we don't know what will happen to astronauts who are in space for years at a time, both physically and mentally). In the next several decades we will probably begin to reach out within our solar system, but it will take economic pressure to establish permanent residences.

As for colonizing other stars, I personally believe that it may take thousands to millions of years, if ever, for us to reach out. The nearest star is four light-years away, but at the speeds of modern rockets, it would take us tens of thousands of years to get there. There are dreamy designs for nuclear powered rockets that may be able to go as fast as 1/10 the speed of light, but even then it would take 40 years to get to the nearest stars. And we have no clue if these stars have planets capable of supporting life! Outer space is much larger than most people imagine, and so I think it will be much longer than most people imagine before we are able to bridge the gaps between stars.

Thanks for writing!

Sincerely, Kurtis Williams

how_long_would_it_take_us_to_travel_to_alpha_centauri

czars — 2009-02-02T13:16:38-08:00

If light travels at 186,000 miles per hour and a star is 75,000 light years away how long would it take for the light from the star to reach us. And how long would it take for a flight to it. Also could you provide formula to your solution.

I'll give you some hints to your problem, but I won't solve your exact problem. I hope that's okay. By the way, light travels at 186,000 miles per SECOND, not per hour.

First of all, a light-year is the distance light travels in one year. The star Alpha Centauri is about 4 light-years away, so light from Alpha Centauri takes 4 years to get to the Earth.

Now, if we wanted to travel to Alpha Centauri, we would have to know how fast our spaceship is travelling. The fastest rockets move at about 20,000 mph. Suppose we launch one at 20,000 miles per hour toward Alpha Centauri. How long would it take to get there?

Well, distance travelled equals speed times time, so the time it takes to travel a distance is that distance divided by the speed, or

4 light-years / (20,000 miles / hour)

Now this isn't terribly illuminating. We need to convert units so that we can do the division. There are 6 trillion miles in a light-year (approximately), so the distance we need to go is 6 trillion miles / light-year times 4 light-years, or 24 trillion miles. So, this trip would take 1.2 billion hours. There are 24 hours a day and 365.25 days per year, so this time in years is 137 thousand years.

That's a long time! Now you can start to see while it is virtually impossible to visit other stars - even the closest star would take hundreds of thousands of years to reach!

Thanks for writing, Kurtis Williams