Author Topic: Ask an astronomer  (Read 53431 times)

oldtone27

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Re: Ask an astronomer
« Reply #30 on: August 02, 2013, 03:49:51 PM »
I was watching the weather forecast yesterday and the spinning globe image they show had a sunset superimposed so that it looked like Jupiter, with its red and orange bands.

This set me thinking. Our temperatures are shown at Earth's surface and from space the surface looks is more or less solid (i.e. land and water) with perhaps a little cloud.

Looking at Jupiter all that can be seem is cloud (probably not water vapour). I have seen temperatures for Jupiter quoted, can't remember what, but to what level would these refer.

Then I thought does Jupiter actually have a surface? Does anyone know or theorise what there is below its clouds?

starsmurf

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Re: Ask an astronomer
« Reply #31 on: August 02, 2013, 11:08:35 PM »
Sorry Tone, I'll answer your questions in order.

Thank you starsmurf.

Firstly no need to apologise starsmurf. Some of this stuff is quite complex so I don't expect a quick answer. In fact I find your reply one of the most understandable summaries I have read. Thank you again for the time and trouble to compile it, you have made the subject a lot clearer.

Thanks Tone  >blush<

Quote
Last week New Scientist had a article about class 1A supernovae which suggested that maybe not all are the same type. It was one that is 'too big' that give pause for thought. The great thing about science is that it keeps scientists on their toes.

There was also a review about a book called "The Universe in the Rearview Mirror" about Emmy Noether who in the early twentieth century had some profound theories about symmetry in physics. I hadn't heard of this lady before but apparently such symmetries are fundamental to much modern physics.


The Type Ia supernovae had always been a standard candle because of how they occur.  I'll attempt to explain them and why they should be the same.

When a star like our Sun dies, it doesn't blow itself apart in a supernova.  A star is a delicate balance between gravity and energy.  A star's energy is produced by fusing hydrogen into helium in the hottest part of the star, the core.  The energy from this reaction pushes outward on the gases that make up the star, while gravity tries to push them inward.



The lifecycle of a star like the Sun is explained in the diagram above.  Most stars are Main Sequence stars, that is, they're fusing hydrogen in their cores.  When the hydrogen runs out, gravity starts to compress the core because there is no energy coming outward.  The star turns into a red giant.  The Sun will be as large as Earth's orbit when this happens and will swallow Mercury, Venus and possibly Earth.



As the core is compressed, the temperature increases.  Fusion requires very high temperatures and this temperature increases for each element.  Hydrogen needs 10 million C to fuse to helium but helium needs a temperature of 100 million C to fuse into carbon.  When the core reaches 100 million C it can start to draw on this new source of energy.

For a low-mass star, anything below eight times the mass of the Sun, that's it.  They'll never reach the temperature required to start fusing carbon.  The core of the star gets compressed while the outer layers drift away, forming a planetary nebula.  The core becomes exposed and is called a white dwarf.  The leftover heat is what makes it shine and the pressure of the electrons in the core stops it collapsing any further.  This object can have the mass of the Sun in a sphere smaller than the Earth.

So what does that have to do with type-Ia supernovae?  I said a small star doesn't blow apart in a supernova.  However, in special circumstances, the white dwarf can.

It requires the white dwarf to have a companion star.  Our Sun is a solitary but most stars exists in pairs (binaries) or triple, quadrulple or even more complex star systems.  Our white dwarf must be in a very close binary.  When the companion star begins to swell into a red giant, the outer layers come so close to the white dwarf that it can begin to attract them by gravity.



If a white dwarf's mass goes over 1.44 times the mass of the Sun, gravity overcome the pressure of the electrons.  Once the white dwarf hits this mass, the core collapses into a neutron star and triggers a supernova, just as it does in larger stars that go supernova immediately.  Because the mass is always the same, type-Ia supernovae are very good standard candles.  They were used to discover the fact that the expansion of the universe is speeding up and those who made that discovery were just awarded the Nobel Prize in Physics.

The New Scientist article suggested that some can exceed that 1.44 solar mass limit, perhaps because of how fast the star is spinning, they would therefore be much brighter.  There is another theory that the 1.44 solar mass limit is never reached and the white dwarf starts to fuse the carbon and oxygen in its core.  This causes the temperature in the white dwarf to rise.  A normal star could expand and cool to overcome this problem but the white dwarf can't.  Because it can't regulate its energy production in this way, it can have a runaway effect that blows the star apart.

So once again in astronomy you have an answer but there are also two more...
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oldtone27

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Re: Ask an astronomer
« Reply #32 on: August 03, 2013, 03:33:33 PM »
Thanks again for your reply starsmurf. Once more another clear explanation.

I was reading that Saturn is using its satellite Enceladus like a puffer. As the satellite orbits Saturn it is squeezed to varying degrees which causes icy water to be ejected from fissures in its surface. Strange the things that go on 'out there'.

I hope this isn't getting to techy for other Ouchers and frightening them off.

devine63

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Re: Ask an astronomer
« Reply #33 on: August 03, 2013, 09:05:57 PM »
Hi Oldtone

I can't recall ever before seeing someone "anthropomorphising" about a planet (I mean you have written about Jupiter as if it has sentience and can have an intention to make the satellite behave in that fashion).   Maybe it's just my limited experience of this topic, but it's amusing!
regards, Deb

oldtone27

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Re: Ask an astronomer
« Reply #34 on: August 04, 2013, 11:44:33 AM »
Although I like to think I have a somewhat analytical mind others have told me I often have an obtuse take on things. I guess we need obtuse folk to make life interesting. That's my excuse anyway.

I think that is the appeal of astronomy, physics, and science in general. Its full of strange and wonderful (literally) things.

Sunshine Meadows

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Re: Ask an astronomer
« Reply #35 on: August 04, 2013, 02:20:09 PM »
I dont know if this questions is actually silly or not but I will ask it anyway.

If a meteorite or comet hit the moon or Earth comparatively speaking would it take a larger/faster object to hit the moon to knock it out of orbit with the Earth than it would for the Earth's orbit round the sun to change?

Is the moon's orbit maintained by its position in the solar system and also by Earth's gravity?


devine63

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Re: Ask an astronomer
« Reply #36 on: August 04, 2013, 08:01:25 PM »
 >thumb-up<  Oldtone

It was just I am used to people attributing human characteristics to their pets, but not to planets!!!

regards, Deb

starsmurf

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Re: Ask an astronomer
« Reply #37 on: August 04, 2013, 11:46:53 PM »
I was watching the weather forecast yesterday and the spinning globe image they show had a sunset superimposed so that it looked like Jupiter, with its red and orange bands.

This set me thinking. Our temperatures are shown at Earth's surface and from space the surface looks is more or less solid (i.e. land and water) with perhaps a little cloud.

Looking at Jupiter all that can be seem is cloud (probably not water vapour). I have seen temperatures for Jupiter quoted, can't remember what, but to what level would these refer.

Then I thought does Jupiter actually have a surface? Does anyone know or theorise what there is below its clouds?

Astronomers can't know for certain what goes on below the clouds of Jupiter but they'll always have ideas.  >biggrin<  These are based on what we know about the planet and the laws of physics.

Jupiter is a type of planet called a gas giant.  It is 11 times the diameter of the Earth and 318 times the mass of the Earth.  The cloud layer is very thin by comparison, only about 30 miles (50km) deep, just as our atmosphere is very thin in comparison to the planet.  The temperture at the top of the clouds is usually used for Jupiter's temperature, it's -145 C.  Once you get roughly 75 miles below the cloudtops the temperature is about 20 C.

The bands we see on Jupiter are weather systems, like our high- and low-pressure systems.  As Jupiter rotates in only 9 hours and 55 minutes, the systems are drawn out into bands by the equivelent of the jet stream.  The white bands are called zones and the brown bands are called belts.  The zones are higher in Jupiter's atmosphere than the belts.



The clouds are made of ammonia cystals and possibly ammonia hyrdosulphide.  The white colour of the zones is due to ammonia ice.  The brown and orange colours of the belts is probably due to chemicals that are being drawn up from lower layers by convection.  These chemicals change colour as they are exposed to ultraviolet light from the Sun.  It's not known with certainty what they are but they might be phosphorus or sulphur.



You can see the structure here of the cloud layers in the diagram above.  There are temperatures given but they're in Kelvin, which can easily be converted to Celcius by substracting 273.  So 300 K is 27 C.  Please note the altitude doesn't mean that Jupiter has a solid surface.  In astronomy, surface doesn't mean something you could stand on, unless it refers to a rocky planet.  Surface refers to the visible surface of the object, so astronomers refer to the surface of the Sun.  In Jupiter's case, it means the top of the cloud layers.

To find out more about these layers, astronomers dropped a probe from the Galileo spacecraft into Jupiter's atmosphere.  Unfortunately, it fell into a hole in the clouds and the measurements were very different to what was expected.  Hopefully, in the future another one will get dropped and we'll get exact measurements.

There are smaller weather systems that don't get drawn out into bands and are simply oval (or elliptical to be precise).  The best known of these is the Great Red Spot, a storm the size of Earth that has been raging for at least 300 years and may be permanent.  There are also smaller white storms which do not last as long.  Three of these storms merged in 2000 and turned red!  This storm is properly called Oval BA but is better known as the Little Red Spot (or Red Spot Jr in the USA).  The red colour may be due to red phosphorus or complex organic molecules (that doesn't mean life!)

Great Red Spot with Earth for comparison, plus white storms


Little Red Spot


There is debate as to whether there is anything solid within Jupiter but even if there is, it could not be called a surface, as it would simply be the core of the planet.  If you're interested as to why that is, please ask.  It's to do with how the Solar System was formed.



The very high pressures within Jupiter compress the hydrogen, so that it exists as a liquid.  The temperature here is about 9,700 C.  There is probably a core made of rock and ices, about the size of Earth but with three to ten times the mass of Earth.  The temperature here is about 35,500 C.  For comparison, Earth's core is at about 6,000 C and the temperature at the surface of the Sun is 5,500 C.

I hope that helps, please ask if you want to know more (including about the formation of Jupiter and the rest of the Solar System).
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starsmurf

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Re: Ask an astronomer
« Reply #38 on: August 05, 2013, 01:10:07 AM »
Thanks again for your reply starsmurf. Once more another clear explanation.

I was reading that Saturn is using its satellite Enceladus like a puffer. As the satellite orbits Saturn it is squeezed to varying degrees which causes icy water to be ejected from fissures in its surface. Strange the things that go on 'out there'.

I hope this isn't getting to techy for other Ouchers and frightening them off.

Thanks oldtone, you're very bad for my humility!  >biggrin<



Enceladus is one of the larger of Saturn's inner satellites.  It orbits about 148,750 miles from the centre of Saturn or 112,500 miles from Saturn's cloudtops.  Enceladus is 307 miles across.  It has a surface of water ice, with a liquid ocean below the surface, which covers a rocky core.  The ice makes it very bright and very cold (-201 C), as it reflects almost 99% of the sunlight that falls on it

Enceladus's close orbit to Saturn causes it to be affected by tides caused by the massive planet's gravity.  If you think about the effect of our small Moon on the tides of Earth's oceans, then imagine the effect on Enceladus's oceans.  Saturn is 95 times the mass of Earth and the two objects are much closer together.  Enceladus's orbit isn't perfectly circular, so the tidal forces change over its orbit.



The tides not only affect Enceladus's oceans but it's rocky core.  This has created a hot spot near the satellite's south pole.  The heat has created geysers.  They spew through cracks, the so-called "tiger stripes", on the icy surface. 



The Cassini spacecraft made an image which shows that these stripes are hotter than the surrounding terrain:



These operate a little like a garden sprinkler.  When the satellite is closest to Saturn, the vents are squeezed shut, when the satellite is furthest from Saturn, the vents are wide open and spew water vapour.



The Cassini spacecraft actually flew through one of the plumes and found that they're made up of mostly water ice with carbon, methane and nitrogen in them.  These plumes are the source of Saturn's outermost ring (which Enceladus orbits within), the E ring:





Sorry about the huge second picture but it was the only pastable one I could find that still clearly shows Enceladus creating the ring (the dark dot above the bright vent)


Space is mind-blowing sometimes.  I guess I get so used to ideas like a 300 mile body of rock getting squeezed by a giant planet's gravity, I forget just how odd an idea it is to most people!  >lol<

>Edited to correct typo<
« Last Edit: August 05, 2013, 02:27:57 AM by starsmurf »
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starsmurf

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Re: Ask an astronomer
« Reply #39 on: August 05, 2013, 01:11:06 AM »
Hi Oldtone

I can't recall ever before seeing someone "anthropomorphising" about a planet (I mean you have written about Jupiter as if it has sentience and can have an intention to make the satellite behave in that fashion).   Maybe it's just my limited experience of this topic, but it's amusing!
regards, Deb

You don't want to read astronomer fan fiction then  >yikes<
 >lol<
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starsmurf

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Re: Ask an astronomer
« Reply #40 on: August 05, 2013, 02:24:18 AM »
I dont know if this questions is actually silly or not but I will ask it anyway.

If a meteorite or comet hit the moon or Earth comparatively speaking would it take a larger/faster object to hit the moon to knock it out of orbit with the Earth than it would for the Earth's orbit round the sun to change?

Is the moon's orbit maintained by its position in the solar system and also by Earth's gravity?

Hi Sunshine,

There are no silly questions in astronomy.  I'll take your questions in order.

It would take a smaller object to affect the Moon than it would to affect the Earth because the Moon is much smaller and therefore it doesn't take as much force to affect it.  It would also take much less force to affect the Moon's orbit around the Earth, than it would to affect the Earth's orbit around the Sun.

Both the Earth and the Moon have been struck by very large objects in the past.  It was an object about the size of Mars (4000 miles across versus 8000 miles for the Earth) hitting the Earth that formed the Moon in the first place.  This all happened in the early days of the Solar System, it would be impossible as things are today. If you want to know how a big impact led to the formation of the Moon, please just ask.



You can see evidence of the end of this violent period on the face of the Moon.  The darker patches on the Moon are huge areas of lava, formed between 4.1 and 3.8 billion years ago.  They were created when huge asteroids, much bigger than anything in the inner Solar System today, struck the Moon.  They were so large that they punched through to the inner part of the Moon that was still molten at that time.  This caused a flood of lava, which filled in these holes.  The largest of these impacts is not visible without a spacecraft, as it is on the far side of the Moon, but it is around 1,600 miles across and over 8 miles deep even with all the lava that filled it.



Astronomers know that there were lots of smaller impacts, as the Moon is covered in craters, which can be seen with any telescope.  We know that the lighter areas of the Moon are younger, because there are more craters there.  The darker patches don't have anywhere near as many.  That tells us that the rate of impacts decreased significantly after the era of these big impacts.



The Moon would've mopped up many of these asteroids but Earth would still have been pummelled by them, with one causing serious enviromental damage every 100 years on average.  For a long time it was thought that the Earth had remained molten until 3.8 billion years ago, as no rocks older than this could be found.  This didn't make sense, as the Earth should've been cool enough to form solid rock much earlier.  This rain of asteroids would explain the difference and it gives a idea of just how intense the bombardment was.

These huge objects weren't being flung about by collisions but because they got too close to a larger object.  This effect is sometimes called the gravitational slingshot effect.  For example, when comets or asteroids get too close to Jupiter, they either get flung out of the solar system or thrown inwards toward the Sun.  Only if they get very close do they hit the planet, like Comet Shoemaker-Levy 9 did in 1994.

If the Earth or Moon were hit by a large, fast-moving object that had enough force to knock it out of its orbit, the force would be so great that the Earth or Moon would be destroyed.  There is nothing in the Solar System large enough to have that effect that isn't a planet or in a safe orbit far away.



The Moon's orbit around the Earth is stable because of the Earth's gravity.  Other objects in the Solar System don't have enough of an effect on the Moon to change its orbit.  The most common image of how gravity works is to think of a rubber sheet as the fabric of space.  The Earth is a large lead ball so it creates a well in the fabric.  That well in the fabric is the Earth's gravity.  The Moon is a marble wizzing around inside this well.  It would take a lot of energy to push the Moon out of that well.

I know the image looks like it would be easy to push the Moon out but it was the best I could find!



Please don't worry about any questions seeming stupid, like I said, there are no stupid questions in astronomy.  You never know where they can lead.  The question "why is the sky dark at night" led to the discovery of the Big Bang and the birth of our universe!
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oldtone27

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Re: Ask an astronomer
« Reply #41 on: August 05, 2013, 10:20:55 AM »
Thanks for you further replies starsmurf.  >thumbsup< Most interesting. Nothing more for now you will be relieved to learn, but I expect something will trigger my grey matter before long.

Dic Penderyn

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Re: Ask an astronomer
« Reply #42 on: August 05, 2013, 12:05:47 PM »
starsmurf maybe you should get up there on the stage in Edinburgh.  >lol<

http://www.bbc.co.uk/news/entertainment-arts-23571166
« Last Edit: August 05, 2013, 12:07:23 PM by Dic Penderyn »
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Yvette

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Re: Ask an astronomer
« Reply #43 on: August 05, 2013, 12:26:16 PM »
Why does the moon shine at night?  Is it reflected light from the sun?

devine63

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Re: Ask an astronomer
« Reply #44 on: August 05, 2013, 12:56:19 PM »
And do I remember correctly that when standing on the moon there are two light sources: sun and earth?
regards, Deb