ccwebb Posted May 7, 2014 Share Posted May 7, 2014 A few days ago I read about this globular cluster of stars coming at us 'like a cannon ball'! (reporters crack me up!) According to the article, there is a cluster of 100,000 stars heading this way from 53 million light years away. They are figuring it will be here in about 17-18 million years. Now please bare with me, I might start talking out of my butt here... but if the cannon ball is traveling at 2 million MPH(according to the article), which is 894 Km/sec, isn't space expanding faster than that with these kind of distances? Expansion increase the further apart items are, right? I get the fact Andromeda Galaxy will reach us, it is only 2.5 ly away. Will this 'cannon ball' ever reach us? Link to comment Share on other sites More sharing options...
Mordred Posted May 7, 2014 Share Posted May 7, 2014 yes expansion is 67.3 km/s/Mpc, your object is moving 894 Km/sec which is faster than the rate of expansion Link to comment Share on other sites More sharing options...
ACG52 Posted May 7, 2014 Share Posted May 7, 2014 And Andromeda is 2.5 million lys away. Expansion doesn't become evident until your reach 200 million lys. Closer than that and gravity overcomes expansion. Link to comment Share on other sites More sharing options...
mathematic Posted May 7, 2014 Share Posted May 7, 2014 A few days ago I read about this globular cluster of stars coming at us 'like a cannon ball'! (reporters crack me up!) According to the article, there is a cluster of 100,000 stars heading this way from 53 million light years away. They are figuring it will be here in about 17-18 million years. Now please bare with me, I might start talking out of my butt here... but if the cannon ball is traveling at 2 million MPH(according to the article), which is 894 Km/sec, isn't space expanding faster than that with these kind of distances? Expansion increase the further apart items are, right? I get the fact Andromeda Galaxy will reach us, it is only 2.5 ly away. Will this 'cannon ball' ever reach us? Statement is crazy. If something is 53 million light years away it can't reach us in 17 million years! 1 Link to comment Share on other sites More sharing options...
Orodruin Posted May 7, 2014 Share Posted May 7, 2014 yes expansion is 67.3 km/s/Mpc, your object is moving 894 Km/sec which is faster than the rate of expansion Did we just compare a rate with a velocity? With the figures he put in: 53 Mly is approximately 16 Mpc, which would mean an expansion velocity of H*d of order 10^3 km/s, i.e., larger than the quoted velocity. This is of course assuming that the quoted velocity is relative to the comoving frame. Of course, it is borderline and the Milky way is also moving wrt the comoving frame which may change things. On the other hand, the quoted velocity is not even in accordance with the stated timings if the Universe was not expanding - if it is 53 Mly away, we see the light now, and the projectile will arrive here in 17ly, then it would need to travel at roughly 0.76c >> 900 km/s. Conclusion: Do not trust things you read in magazines blindly, they have put numbers that do not add up without telling you how they arrived at them. Statement is crazy. If something is 53 million light years away it can't reach us in 17 million years! We see it 53 Mly away, which means that we see it as it was 53 Myr ago and that its total travel time would be 70 Myr. 53/70 ~0.76 < 1. 2 Link to comment Share on other sites More sharing options...
Mordred Posted May 7, 2014 Share Posted May 7, 2014 Conclusion: Do not trust things you read in magazines blindly, they have put numbers that do not add up without telling you how they arrived at them good point, Link to comment Share on other sites More sharing options...
ccwebb Posted May 8, 2014 Author Share Posted May 8, 2014 What... media has something wrong? Again? "Why Ike, whatever do you mean?" Good point Orodruin! And Andromeda is 2.5 million lys away. Expansion doesn't become evident until your reach 200 million lys. Closer than that and gravity overcomes expansion. I thought I understood expansion to be similar to compounding interest. For every Megaparasec the expansion is increased, again? Why would expansion not be evident until 200 million LY? Link to comment Share on other sites More sharing options...
Greg H. Posted May 8, 2014 Share Posted May 8, 2014 Why would expansion not be evident until 200 million LY? Because (as I understand it) at distances less than that amount, the force of expansion is smaller than the force of gravity Link to comment Share on other sites More sharing options...
Sensei Posted May 9, 2014 Share Posted May 9, 2014 Each light year is approximately299792.458 * 60 * 60 * 24 * 365.25 = 9.46*10^12 km53.0 million light years * 9.46*10^12 = 5*10^20 km distance between us.3.2 mln km/h = 888.9 km/s5*10^20 km / 888.9 km/s = 5.641*10^17 secondsThat's 1.78*10^10 years.17.8 billion years.Not millions.Article is wrong by factor 1000. Not first and not last time journalist shows his inability to calculate."First of all, even at its current speed, it will take more than 17.5 million years to get here." Link to comment Share on other sites More sharing options...
Mordred Posted May 9, 2014 Share Posted May 9, 2014 here is the arxiv paper on it http://arxiv.org/pdf/1402.6319v2.pdf I seem to be having some trouble getting it to load though found this article as well, I was trying to locate its proper distance http://www.universetoday.com/111609/runaway-star-cluster-breaks-free-from-distant-galaxy/ according to this article and that I couldn't find it in the arxiv article "The star cluster is moving so fast it should soon by sailing into intergalactic space. It may already be, but its distance remains unknown." its still too early to tell as we don't have its proper distance Link to comment Share on other sites More sharing options...
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