Starship Asterisk*

Spiral Galaxy NGC 6744

Explanation: Beautiful spiral galaxy NGC 6744 is nearly 175,000 light-years across, larger than our own Milky Way. It lies some 30 million light-years distant in the southern constellation Pavo and appears as only a faint, extended object in small telescopes. We see the disk of the nearby island universe tilted towards our line of sight in this remarkably detailed galaxy portrait, a telescopic view that spans an area about the angular size of a full moon. In it, the giant galaxy's elongated yellowish core is dominated by the light from old, cool stars. Beyond the core, grand spiral arms are filled with young blue star clusters and speckled with pinkish star forming regions. An extended arm sweeps past a smaller satellite galaxy (NGC 6744A) at the lower right. NGC 6744's galactic companion is reminiscent of the Milky Way's satellite galaxy the Large Magellanic Cloud.

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A bit tangential question/topic - Do we really know how big is our own milky-way galaxy and how many stars and planets there are ? There is so much of ambiguity or variance, it feels it's such a wide guess (at times giving impression of such *wild* guess).. Some reliable sites say diameter is 100,000 ly, some say 150,000 ly, some say 200,000 ly.. Some sites say stars are 250 billion with +/- 150 billions.. Whaaaat ? If the variance is 5-10% then we can say we know reasonably well.. but with 50-100% variance, it feels it's wild. And, planets - they say around 100 billion+. Seriously ? How do we know ? Do we even know ? Are we saying on an average each star doesn't have even 1 planet (average, approximately) ? Finding planets is SOOOOOOOOOOO difficult, and here, someone claims there are 150 billions.. Difficult to believe these #s, right ? What do you guys think ? It's so unfortunate that it'll take maybe 1000 more years for humans (or whatever species it's called then) to send the spaceship/telescope outside of this galaxy and actually see/measure and validate what we currently are assuming or guessing.. No ? Thanks in advance for everyone's help/comments.

Nice work...

:---[===] *

An extended arm sweeps past a smaller satellite galaxy (NGC 6744A) at the lower right.

This arm swings out quite a bit from the galaxy; but you can almost see light dust clouds between this arm and the main body! Or is it my imagination?

Not sure how usual this is — I don't see many deep background galaxies, apart from a few small edge-on spirals and ellipticals off in the top left corner. I always thought it was difficult to point a telescope in a random direction and not catch a lot of veryverylongwayaway stuff (as in the various Hubble Deep Field images). Or... could the telescope used simply not be capable of catching whatever extreme deep background objects there are in the field of view?

orin stepanek wrote: ↑Thu Dec 05, 2019 12:24 pm

An extended arm sweeps past a smaller satellite galaxy (NGC 6744A) at the lower right.

This arm swings out quite a bit from the galaxy; but you can almost see light dust clouds between this arm and the main body! Or is it my imagination?
NGC6744_FinalLiuYuhang1024.jpg

There are definitely visible dust lanes to the right of the core. In fact, we expect the presence of dust lanes in all the major arms.

Ann

TheOtherBruce wrote: ↑Thu Dec 05, 2019 1:28 pm
Not sure how usual this is — I don't see many deep background galaxies, apart from a few small edge-on spirals and ellipticals off in the top left corner. I always thought it was difficult to point a telescope in a random direction and not catch a lot of veryverylongwayaway stuff (as in the various Hubble Deep Field images). Or... could the telescope used simply not be capable of catching whatever extreme deep background objects there are in the field of view?

A noted: this is a relatively large bright galaxy...so any background galaxies would generally be intrinsically smaller & dimmer.

If the APOD had instead zeroed in on it's small dim companion galaxy (NGC 6744A) then it would have been much easier to detect more distant intrinsically larger & brighter background galaxies.

TheOtherBruce wrote: ↑Thu Dec 05, 2019 1:28 pm Not sure how usual this is — I don't see many deep background galaxies, apart from a few small edge-on spirals and ellipticals off in the top left corner. I always thought it was difficult to point a telescope in a random direction and not catch a lot of veryverylongwayaway stuff (as in the various Hubble Deep Field images). Or... could the telescope used simply not be capable of catching whatever extreme deep background objects there are in the field of view?

Interesting question. My impression is that there are quite a lot of background stars in this part of the sky. Also NGC 6744 is an intrinsically large, bright and also comparatively nearby galaxy, which means that most background galaxies are totally dwarfed by NGC 6744.

The fine picture by David Fitz-Henry at right shows four important background galaxies of NGC 6744. Look at the caption under the picture to find out more.

I strongly recommend this interactive page which shows many background galaxies in the same part of the sky as NGC 6744. As you can see, however, all the background galaxies are quite faint and small compared with NGC 6744.

Ann

Since we're dealing in tangentials, might I go back to the core. There was a time when a meter was represented by a metal bar kept in a safe in Paris. Technology then made possible that the time light travelled in a meter was the standard measure of distance. So why do we persist in describing galaxies as being such and such distance away and not use the time reference of ago?
With the Universe expanding, doesn't it make sense to use a fixed time measure reference rather than an antiquated distance (metal bar) variable?

DL MARTIN wrote: ↑Thu Dec 05, 2019 3:41 pm Since we're dealing in tangentials, might I go back to the core. There was a time when a meter was represented by a metal bar kept in a safe in Paris. Technology then made possible that the time light travelled in a meter was the standard measure of distance. So why do we persist in describing galaxies as being such and such distance away and not use the time reference of ago?
With the Universe expanding, doesn't it make sense to use a fixed time measure reference rather than an antiquated distance (metal bar) variable?

For nearby objects like this, simple distance units are usually the best way of describing location. But for most objects in the Universe, astronomers use the value of cosmological redshift (z) to describe distance, and z is most often converted to light travel time, not to units of distance.

There is no value in considering light travel time for nearby objects, because the time has not been significantly altered by cosmological expansion. That is, light travel time and distance in meters are the same. It only becomes more complicated when cosmological distances are involved.

Chris Peterson says that things nearby are described as away but I'm sure I've seen references to things being hundred of millions of light years away if not further.

DL MARTIN wrote: ↑Thu Dec 05, 2019 6:00 pm Chris Peterson says that things nearby are described as away but I'm sure I've seen references to things being hundred of millions of light years away if not further.

Sure, because that's the most obvious way to express it to a lay audience (and when they say hundreds of millions of light years, what they're usually taking that from is hundreds of millions of years of light travel time). But in scientific publications, cosmological distance is almost always expressed in terms of redshift. Redshift is a direct measurement; distance is always a secondary inference. A hundred years from now, the value of redshift will be unchanged, but the physical distance or light travel time assigned to that redshift may well have changed.

shaileshs wrote: ↑Thu Dec 05, 2019 7:31 am A bit tangential question/topic - Do we really know how big is our own milky-way galaxy and how many stars and planets there are ? There is so much of ambiguity or variance, it feels it's such a wide guess (at times giving impression of such *wild* guess).. Some reliable sites say diameter is 100,000 ly, some say 150,000 ly, some say 200,000 ly.. Some sites say stars are 250 billion with +/- 150 billions.. Whaaaat ? If the variance is 5-10% then we can say we know reasonably well.. but with 50-100% variance, it feels it's wild. And, planets - they say around 100 billion+. Seriously ? How do we know ? Do we even know ? Are we saying on an average each star doesn't have even 1 planet (average, approximately) ? Finding planets is SOOOOOOOOOOO difficult, and here, someone claims there are 150 billions.. Difficult to believe these #s, right ? What do you guys think ? It's so unfortunate that it'll take maybe 1000 more years for humans (or whatever species it's called then) to send the spaceship/telescope outside of this galaxy and actually see/measure and validate what we currently are assuming or guessing.. No ? Thanks in advance for everyone's help/comments.

This is not all that tangential shaileshs, at least imo. Almost every discussion of this or that galaxy begins with some comparison to the size of the Milky Way. For example, the first sentence in this APOD's explanation says:

Beautiful spiral galaxy NGC 6744 is nearly 175,000 light-years across, larger than our own Milky Way.

But how certain are we about the MW's size? The wikipedia article on the Milky Way states this:

The Milky Way is a barred spiral galaxy with a diameter between 150,000 and 200,000 light-years (ly).[22][23][24][25] It is estimated to contain 100–400 billion stars[26][27] and more than 100 billion planets.[28][29]

Therefore it doesn't seem that it is certain that NGC 6744 is in fact larger that the Milky Way. There still is it seems great uncertainty in the true size and number of stars in our galaxy. And as for planets, we don't even know for sure how many planets there are in our own solar system!

Bruce

This picture is a duplicate from August 10th, 2018 - including identical comments.

kirchner36 wrote: ↑Thu Dec 05, 2019 9:02 pm
This picture is a duplicate from August 10th, 2018
- including identical comments.

shaileshs wrote: ↑Thu Dec 05, 2019 7:31 am A bit tangential question/topic - Do we really know how big is our own milky-way galaxy and how many stars and planets there are ? There is so much of ambiguity or variance, it feels it's such a wide guess (at times giving impression of such *wild* guess).. Some reliable sites say diameter is 100,000 ly, some say 150,000 ly, some say 200,000 ly.. Some sites say stars are 250 billion with +/- 150 billions.. Whaaaat ? If the variance is 5-10% then we can say we know reasonably well.. but with 50-100% variance, it feels it's wild. And, planets - they say around 100 billion+. Seriously ? How do we know ? Do we even know ? Are we saying on an average each star doesn't have even 1 planet (average, approximately) ? Finding planets is SOOOOOOOOOOO difficult, and here, someone claims there are 150 billions.. Difficult to believe these #s, right ? What do you guys think ? It's so unfortunate that it'll take maybe 1000 more years for humans (or whatever species it's called then) to send the spaceship/telescope outside of this galaxy and actually see/measure and validate what we currently are assuming or guessing.. No ? Thanks in advance for everyone's help/comments.

Thanks BDaniel for your response/comments. I was expecting a few more comments from many experts in this group who are normally very gracious to respond/comment giving explanation or their perspective. Makes me think that either my questions are not that easy to answer or folks are a bit busy to respond.. I'll keep coming back to check if anyone else has responded.

shaileshs wrote: ↑Thu Dec 05, 2019 9:57 pm

shaileshs wrote: ↑Thu Dec 05, 2019 7:31 am A bit tangential question/topic - Do we really know how big is our own milky-way galaxy and how many stars and planets there are ? There is so much of ambiguity or variance, it feels it's such a wide guess (at times giving impression of such *wild* guess).. Some reliable sites say diameter is 100,000 ly, some say 150,000 ly, some say 200,000 ly.. Some sites say stars are 250 billion with +/- 150 billions.. Whaaaat ? If the variance is 5-10% then we can say we know reasonably well.. but with 50-100% variance, it feels it's wild. And, planets - they say around 100 billion+. Seriously ? How do we know ? Do we even know ? Are we saying on an average each star doesn't have even 1 planet (average, approximately) ? Finding planets is SOOOOOOOOOOO difficult, and here, someone claims there are 150 billions.. Difficult to believe these #s, right ? What do you guys think ? It's so unfortunate that it'll take maybe 1000 more years for humans (or whatever species it's called then) to send the spaceship/telescope outside of this galaxy and actually see/measure and validate what we currently are assuming or guessing.. No ? Thanks in advance for everyone's help/comments.

Thanks BDaniel for your response/comments. I was expecting a few more comments from many experts in this group who are normally very gracious to respond/comment giving explanation or their perspective. Makes me think that either my questions are not that easy to answer or folks are a bit busy to respond.. I'll keep coming back to check if anyone else has responded.

It's a bit of both I'd guess.

In decades past the 100 billion stars in the MW was often reported, then 200 billion, and now the figure 400 billion is sometimes claimed. Over my lifetime there has been considerable improvement in telescopes and the ability to detect and count dimmer and dimmer stars. It has been discovered that the dim Red Dwarf stars vastly outnumber the bright stars that are easy to detect. Thus the estimated star counts keep increasing.

As for planet counts, my guess would be that even 150 billion will prove to be a gross under-estimate for the total planet count in the Milky Way. Planets are a natural by-product of star formation. If you also count planets expelled from their birth systems there could be dozens of planets per every star in this galaxy. Tens of trillions is more likely than a mere 150 billion!

Bruce

shaileshs wrote: ↑Thu Dec 05, 2019 7:31 am A bit tangential question/topic - Do we really know how big is our own milky-way galaxy and how many stars and planets there are ? There is so much of ambiguity or variance, it feels it's such a wide guess (at times giving impression of such *wild* guess).. Some reliable sites say diameter is 100,000 ly, some say 150,000 ly, some say 200,000 ly.. Some sites say stars are 250 billion with +/- 150 billions.. Whaaaat ? If the variance is 5-10% then we can say we know reasonably well.. but with 50-100% variance, it feels it's wild. And, planets - they say around 100 billion+. Seriously ? How do we know ? Do we even know ? Are we saying on an average each star doesn't have even 1 planet (average, approximately) ? Finding planets is SOOOOOOOOOOO difficult, and here, someone claims there are 150 billions.. Difficult to believe these #s, right ? What do you guys think ? It's so unfortunate that it'll take maybe 1000 more years for humans (or whatever species it's called then) to send the spaceship/telescope outside of this galaxy and actually see/measure and validate what we currently are assuming or guessing.. No ? Thanks in advance for everyone's help/comments.

We're not very well situated to see the contents of our own galaxy. So a large uncertainty in star count is not unreasonable. The planet estimate is based on the statistics of what we've observed, along with theory suggesting that most of the stars in the galaxy probably don't have planets because they're in regions of high density, where gravitational perturbations make star systems unstable. Of course, those systems probably formed planets, then lost them. But estimates for planet counts usually are about those in orbit around stars, not the much larger number of possible rogue planets flying around the galaxy unattached to their parent stars.

BDanielMayfield wrote: ↑Thu Dec 05, 2019 10:26 pm ... If you also count planets expelled from their birth systems there could be dozens of planets per every star in this galaxy. Tens of trillions is more likely than a mere 150 billion!
Bruce

Chris Peterson wrote: ↑Thu Dec 05, 2019 10:36 pm ... But estimates for planet counts usually are about those in orbit around stars, not the much larger number of possible rogue planets flying around the galaxy unattached to their parent stars.

Hmmm ... I wonder if rogue planets are a dime a dozen.
From your comments, I would then guess that there might be hundreds of Earth analogues somewhere around a typical globular cluster, though they would all be frozen (waiting for us to heat and serve).

Question: Is the Big Bang away or ago?

DL MARTIN wrote: ↑Fri Dec 06, 2019 1:42 am Question: Is the Big Bang away or ago?

It happened 13.7 bilion years ago. The edge of the observable universe is slightly under 13.7 billion light travel years away. That horizon is now about 47 billion light years distant. Or, it has a redshift of 1100. These are all different ways of saying the same thing.

MarkBour wrote: ↑Fri Dec 06, 2019 1:06 am

BDanielMayfield wrote: ↑Thu Dec 05, 2019 10:26 pm ... If you also count planets expelled from their birth systems there could be dozens of planets per every star in this galaxy. Tens of trillions is more likely than a mere 150 billion!
Bruce

Chris Peterson wrote: ↑Thu Dec 05, 2019 10:36 pm ... But estimates for planet counts usually are about those in orbit around stars, not the much larger number of possible rogue planets flying around the galaxy unattached to their parent stars.

Hmmm ... I wonder if rogue planets are a dime a dozen.
From your comments, I would then guess that there might be hundreds of Earth analogues somewhere around a typical globular cluster, though they would all be frozen (waiting for us to heat and serve).

Rogue planets probably are very common, but perhaps not nearly as common in association with globular clusters. The stars of globulars are very old and thus are very metal poor, since the universe hadn't had much time to enrich the interstellar medium with elements beyond Li by the time the globulars were forming. Gas giant planets could have formed and been ejected, but it's not likely that many rocky planets could have formed inside GCs.

Chris Peterson wrote: ↑Thu Dec 05, 2019 10:36 pm

shaileshs wrote: ↑Thu Dec 05, 2019 7:31 am A bit tangential question/topic - Do we really know how big is our own milky-way galaxy and how many stars and planets there are ? There is so much of ambiguity or variance, it feels it's such a wide guess (at times giving impression of such *wild* guess).. Some reliable sites say diameter is 100,000 ly, some say 150,000 ly, some say 200,000 ly.. Some sites say stars are 250 billion with +/- 150 billions.. Whaaaat ? If the variance is 5-10% then we can say we know reasonably well.. but with 50-100% variance, it feels it's wild. And, planets - they say around 100 billion+. Seriously ? How do we know ? Do we even know ? Are we saying on an average each star doesn't have even 1 planet (average, approximately) ? Finding planets is SOOOOOOOOOOO difficult, and here, someone claims there are 150 billions.. Difficult to believe these #s, right ? What do you guys think ? It's so unfortunate that it'll take maybe 1000 more years for humans (or whatever species it's called then) to send the spaceship/telescope outside of this galaxy and actually see/measure and validate what we currently are assuming or guessing.. No ? Thanks in advance for everyone's help/comments.

We're not very well situated to see the contents of our own galaxy. So a large uncertainty in star count is not unreasonable. The planet estimate is based on the statistics of what we've observed, along with theory suggesting that most of the stars in the galaxy probably don't have planets because they're in regions of high density, where gravitational perturbations make star systems unstable. Of course, those systems probably formed planets, then lost them. But estimates for planet counts usually are about those in orbit around stars, not the much larger number of possible rogue planets flying around the galaxy unattached to their parent stars.

I'm going to guess that Gaia will give us a better understanding of the size of our galaxy.

But I think that the term itself, "size", is problematic here. What do we mean by a galaxy's size? I think the word might refer to two things, the size of the optically bright disk, and the size of the low-luminosity galactic halo. (Not the the halo itself is all homogeneous.) When the caption says that the diameter of NGC 6744 is 175,000 light-years, it probably refers to the size of the visible disk of NGC 6744. I think most estimates of the size of the Milky Way agree that its visible disk is not as large as 175,000 light-years across.























As Chris said, we can get a much, much better look at Andromeda's halo than at the Milky Way's halo. Andromeda has two halos, one relatively small and starry (but still faint) and one huge and gaseous. The Youtube video allows you to see the nature of the halo at different distances from Andromeda's luminous disk. The closer you are to the disk, the "starrier" is the halo. The last cutout seen in the video, farthest from the disk of Andromeda, appears to be strongly dominated by background galaxies. But the gaseous halo of Andromeda stretches far, far beyond the point where the starry halo appears to end. The light from background quasars are used to determine the size of the gaseous halo of Andromeda.

Hopefully Gaia will give us a better estimate of the size of the visible disk of the Milky Way. Just maybe Gaia can also give us an idea of the size of our galaxy's "starry halo".

Ann

BDanielMayfield wrote: ↑Fri Dec 06, 2019 5:27 am

MarkBour wrote: ↑Fri Dec 06, 2019 1:06 am

BDanielMayfield wrote: ↑Thu Dec 05, 2019 10:26 pm ... If you also count planets expelled from their birth systems there could be dozens of planets per every star in this galaxy. Tens of trillions is more likely than a mere 150 billion!
Bruce

Chris Peterson wrote: ↑Thu Dec 05, 2019 10:36 pm ... But estimates for planet counts usually are about those in orbit around stars, not the much larger number of possible rogue planets flying around the galaxy unattached to their parent stars.

Hmmm ... I wonder if rogue planets are a dime a dozen.
From your comments, I would then guess that there might be hundreds of Earth analogues somewhere around a typical globular cluster, though they would all be frozen (waiting for us to heat and serve).

Rogue planets probably are very common, but perhaps not nearly as common in association with globular clusters. The stars of globulars are very old and thus are very metal poor, since the universe hadn't had much time to enrich the interstellar medium with elements beyond Li by the time the globulars were forming. Gas giant planets could have formed and been ejected, but it's not likely that many rocky planets could have formed inside GCs.

Also bear in mind that it must be much, much easier for (especially low-mass) stars to be ejected from globular clusters than from the Milky Way altogether. Consider the depth of the Milky Way gravitational well! A star must be given a mighty kick indeed to be ejected out of the Milky Way itself. I don't believe that there are that many planets that are slingshotted out of our galaxy, either.

Ann

shaileshs wrote: ↑Thu Dec 05, 2019 9:57 pm

shaileshs wrote: ↑Thu Dec 05, 2019 7:31 am A bit tangential question/topic - Do we really know how big is our own milky-way galaxy and how many stars and planets there are ? There is so much of ambiguity or variance, it feels it's such a wide guess (at times giving impression of such *wild* guess).. Some reliable sites say diameter is 100,000 ly, some say 150,000 ly, some say 200,000 ly.. Some sites say stars are 250 billion with +/- 150 billions.. Whaaaat ? If the variance is 5-10% then we can say we know reasonably well.. but with 50-100% variance, it feels it's wild. And, planets - they say around 100 billion+. Seriously ? How do we know ? Do we even know ? Are we saying on an average each star doesn't have even 1 planet (average, approximately) ? Finding planets is SOOOOOOOOOOO difficult, and here, someone claims there are 150 billions.. Difficult to believe these #s, right ? What do you guys think ? It's so unfortunate that it'll take maybe 1000 more years for humans (or whatever species it's called then) to send the spaceship/telescope outside of this galaxy and actually see/measure and validate what we currently are assuming or guessing.. No ? Thanks in advance for everyone's help/comments.

Thanks BDaniel for your response/comments. I was expecting a few more comments from many experts in this group who are normally very gracious to respond/comment giving explanation or their perspective. Makes me think that either my questions are not that easy to answer or folks are a bit busy to respond.. I'll keep coming back to check if anyone else has responded.

Thank you Chris, BDaniel and Ann for sharing your knowledge and perspective.