2 Q's

[DQuaN]

First, I was discussing diesels with a mate the other day, and about how they are low revving. He said that diesel engines can quite safely rev to about 9k rpm, they just have no power there. I always thought they would blow. Is he right?

Second, i looked at a picture of the Milky Way earlier and thought about how bright it was in the center. I'm not sure if anyone knows the answer to this, but does that mean if we lived on a planet that orbited a star near the center of the galaxy, at night the sky would still be very bright?

Just some wonderings!

 

[wfooshee]

I can't find any facts, but I seem to recall seeing or hearing that the Audi R10 diesel engine turned about 5500 RPM, which is the highest I've ever heard of for a diesel. If I understand the workings correctly, diesels tend to be very long stroke engines compared to gasoline, and long stroke does not lend itself to high revs. Remember you're looking for a seriously high compression ratio.

As for the galactic center the density of stars in the central are is far higher than our podunk rim of the galaxy. I would think that the difference between night and day would be slim, never actually achieving darkness. It probably wouldn't be noonday brilliance, but the number of objects in the sky and their luminosity would be staggering. You'd also have celestial objects bright enough to be visible in the daytime. who knows, you might not even realize which star is your own star without a good education! If you could live through the radiation present from all those stars.

 

[Touring Mars]

I was trying to figure out what it might look like near the centre of the galaxy as compared to here, and was getting mighty baffled by numbers, but came to the conclusion that it is going to be pretty damn bright. The density of stars in the region where we are is only about 0.003 per cubic light year. This number makes sense, given that our nearest neighbour is more than 4 light years away... This means you can draw a sphere around the Sun with a radius of 4 light years, and not find a single other star in that volume, a volume of about 300 cubic light years (i.e. 1 star per 300 cubic light years is about 0.003 stars per cubic light year.)

Some estimates put the star density near the centre of the galaxy at about a million times higher than the density around here. So if you put our solar system in a region with this density of stars, the average distance between stars would be about 100 times less than it is where we actually are, but that would still leave about 400 000 000 000 km between stars (50 times the distance between Pluto and the Sun, and more than 2500 times the distance between Earth and the Sun).

Of course, at that density, and uniformally arranged on an x,y,z grid (unlikely to be the case in practice!), we could expect to find 6 stars at just 0.04 lyr away, over 100 within 0.1 lyr, and 3000 within 0.5 lyrs. The 100 brightest stars we can see in our current night sky lie between 4 and 3000 lyrs away...

 

[TheCracker]

I was trying to figure out what it might look like near the centre of the galaxy as compared to here, and was getting mighty baffled by numbers, but came to the conclusion that it is going to be pretty damn bright. The density of stars in the region where we are is only about 0.003 per cubic light year. This number makes sense, given that our nearest neighbour is more than 4 light years away... This means you can draw a sphere around the Sun with a radius of 4 light years, and not find a single other star in that volume, a volume of about 300 cubic light years (i.e. 1 star per 300 cubic light years is about 0.003 stars per cubic light year.)

Some estimates put the star density near the centre of the galaxy at about a million times higher than the density around here. So if you put our solar system in a region with this density of stars, the average distance between stars would be about 100 times less than it is where we actually are, but that would still leave about 400 000 000 000 km between stars (50 times the distance between Pluto and the Sun, and more than 2500 times the distance between Earth and the Sun).

It's a tricky situation to try and comprehend, but my guess is that the planet would still have distinct day and night sessions. The only real difference would be the amount of stars in the sky. Even if the night sky was jam packed full of stars i shouldn't think that it would make much difference to the amount of light 'useable' - the sky would merely be brighter to look at. Your own sun would still provide a majority of the light energy for your planet - night time would still be 'dark' when the planet's surface is pointing away from that sun. Even if you have nearby stars 100 times closer to our own nearby stars, would they still appear just as pin pricks of light? - i should think so.

 

[Touring Mars]

I think you'd be right - as brightness varies with the square of distance, we could expect the closest stars (assuming they were roughly the same intrinsic brightness as the Sun) to appear as bright (give or take an order of magnitude or three) as the Moon, which although clearly visible during daylight on a clear day, is nothing compared to the brightness of the Sun. Although, with such a high density of stars, it is likely that a few stars would appear very brightly in the daylit sky.

However, many stars are much more luminous than the Sun, and if one were to be nearby, it would be considerably brighter than the moon. If a star a million times brighter than the Sun was situated anywhere within a cubic light year of the Sun, (i.e. anywhere up to 0.5 lyr in any direction from here, or a maximum of 25,000 times further away), it's distance would make it about half a billion times fainter, but it's intrinsic brightness would give it an apparent brightness of a perhaps just one thousandth that of the Sun, hundreds of times brighter than a full moon.

You could probably work out the likelihood of getting such a bright star in a population of a million, but I would have thought that it would be pretty unlikely to find such a powerful star in such a relatively small population.

 

[Jay]

I can't find any facts, but I seem to recall seeing or hearing that the Audi R10 diesel engine turned about 5500 RPM, which is the highest I've ever heard of for a diesel. If I understand the workings correctly, diesels tend to be very long stroke engines compared to gasoline, and long stroke does not lend itself to high revs. Remember you're looking for a seriously high compression ratio.

Diesel fuel burn rate is much longer than regular fuels, it simply doesn't burn fast enough to allow high revs.

 

[Keef]

Diesel fuel burn rate is much longer than regular fuels, it simply doesn't burn fast enough to allow high revs.

Right. It burns so slowly that at 9000 rpm, like Dunc's friend said for instance, the fuel wouldn't hardly have time to burn at all. Basically, there'd be a point where the engine would simply stop going faster, and start misfiring and spewing fuel out the exhaust. One of the reasons diesels have such tremendous torque within a very tight rev range is because that range is where the fuel is burning most completely from the piston's top dead center to bottom dead center. Beyond that range a diesel's poewr and torque drops off very rapidly.

 

[niky]

It might reach higher revs given more precise injection... but by that point, your injectors would be so powerful, they could probably combust diesel just by the mere act of injection...

At the center of the milky way? Do we mean just outside the center, or in the middle of that big black hole down there?