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so today i want to talk about messier seven okay which is the lowest numbered messier that we still have to do so we're crossing off all the tens by doing messier seven which is very exciting it's a open cluster of stars it's right at the end of the tale of scorpius the constellation of scorpius and it's actually the most southerly messier object as well it's actually been known since antiquity this cluster one of the earliest recorded notations of it being seen i guess whatever you want to call it was in 130 a.d by ptolemy how do i pronounce his name i haven't said that out loud before someone will correct me in the comments if that is completely wrong i'm sure it's about 80 stars in messier 7. so it's
not the biggest we're not the smallest of clusters either so it's about 980 light years away so that means that generally its actual size is about 25 light years across and there's a lot bigger stars than the sun in this cluster that's usually what happens with clusters when you know form a lot of stars all at once from the same cloud of gas you end up with bigger stars than the sun usually just because a lot of stuff's been forced together so the total mass of the cluster is probably something like seven to eight hundred times the mass of the sun what i wanna talk about with messier 7 though and why i think that it is quite special in some regards is because the journey to pluto
wouldn't have been possible without messier 7. the imager on the new horizons spacecraft that visited pluto was all calibrated on messier 7. so messier 7 was actually the very first thing that the long-range reconnaissance imager or lorry on new horizons actually took an image of m7 essentially what they have to do when you launch a giant spacecraft into space obviously you test everything in the lab before you do it you check everything is working you know the cameras are working all the hardware is working but then you put it on a giant rocket and you send it into space and it obviously gets shaken up a lot so one of the first things you do after you sort of set it on the right trajectory
and it's you know going towards pluto is you have to check that all of your instruments are working but also you have to check that your hardware and your camera are working as well and so actually lorry was the last thing to be tested on new horizons which i think just a big gamble leaving your camera to the last thing to be tested because you don't want to go all the way to pluto and not take an image so what they did was uh first thing obviously was open sort of the lens cap if you will it was kind of like a hinge door that was covering laurie and then five minutes later they finally got an image back from laurie sent back to earth and steve conard who was like the lead of the
engineering team was recorded to have shouted i see stars i was finding lots of blogs from the pi of the mission the principal investigator you know alan stern about oh we're doing this commissioning test now and oh we're testing it two years later now and because they obviously had to do it periodically because it was a really long journey to pluto it was something like you know seven eight nine years to get to pluto and so they had to check it periodically and always used messier 7 to do that and so i thought how am i going to find out why they chose messier 7. so i turned to twitter obviously and i tweeted alan stood and i said hi alan i'm prepping a video on messier 7 for deep sky videos and i can't find anything about why m7 was
specifically picked for first light and all the calibration images for laurie can you remember why this was a long time ago that they would have chosen messier 7. i was like he might not even remember he was like contact hal weaver at apl he's actually the laurie instrument pi so he was the head of the whole of the lorry instrument division so he would probably remember so then i emailed hal weaver and how weaver told me why they selected messier seven which was really good of him by the way he literally responded within two hours and i was like thanks hell that's really helpful it's almost like he was waiting for yourself obviously he was just waiting for the day that someone desperately needed to know why messier 7 was picked and the
technical reasons that i could think of hal did confirm those he said that essentially there was a good number of stars that lori could detect with its sensitivity about 25 or so and they were spread out across the whole field so it meant that you could test whether the whole camera was actually working and then also the fact that they were bright enough this is actually only about eight arc seconds across very small region indeed and that was just literally because that was the detector size that they had so they could see the very center of the cluster and that went the next time they pointed it they could check whether the stars were in the same place where they were getting the same
brightness across the detector which is really quite nice it's not a very impressive image but this is the first image that new horizons took before it went on to take images of jupiter when it flew past jupiter and then images of pluto and now even images of an asteroid in the kuiper belt as well right like the weird snowman image that they sent back on new year's day that was really cool but one of the reasons that hal said that they chose m7 which i didn't actually think of in the first place was that the solar elongation angle was greater than 90 degrees which is essentially a really fancy way of saying the sun was behind them or not in the way of the image otherwise
you would have got a load of scattered sunlight on all those images and that could have obviously saturated some of the pixels as well which would have been really bad for the camera elongation angles in general is something we tend to put to planets rather than the sun and it's a really nice simple way of thinking about where things sort of lay in relation to each other so if we picture the solar system and we have this is my very rudimentary sun and we imagine sort of here's the earth and it's going round the sun in a kind of a circle if we have an inner planet going around the sun as well say maybe venus or mercury we can imagine the angle between all of these planets so say if mercury is over here in its
orbit then the angle with respect to the sun will be this angle and that will be the elongation angle of mercury is how distant is from the sun you can obviously have mercury over here in which case your angle will be over this side or it could be further around in which case it would be a much smaller angle so if you've got new horizons moving towards pluto clearly away from earth away from the sun essentially what you're going to want to do is keep the sun behind you and you're going to want that angle to be greater than 90 degrees and so what that would look like is this is clearly not to scale but if we had our little new horizons craft here on its way to pluto and say messier 7 is
you know a star cluster over here essentially what that means is that they're going to want the elongation angle which is if you connect the sun new horizons and messier 7 on the long this line then you're going to want that to be greater than 90 degrees and how i've drawn it that angle is 180 degrees right and that's the ideal that's where you'd always want the sun if you could have it completely behind you but what they essentially say is the reason they picked it is that for most of the journey of new horizons to pluto messier 7 essentially was at an angle of greater than 90 degrees so it could have been over here or it could have been over here it could have been in any of these
this region here on new horizons journey and so without it they wouldn't have had you know well they wouldn't have been able to say that their camera was working it would have been very difficult for them to pick another target that was so well known and so bright and so with the stars so structured across the field as well and having that recognizable distribution that you could know that your camera was working properly saying that however in 2013 they switched targets because there was another cluster that at that point became a better target so it was probably in a better region of solar elongation angle and it also had more stars as well for them to get into the field and that was called the wishing well cluster this is also ngc 3532
this has got 150 stars in it bit further away though 1 320 light years rather than just less than 1 000 that messier 7 is but this image is actually a really famous image because it is actually one of the furthest images ever taken from earth so this was taken at 3.79 billion miles away from earth in terms of astronomical units which is the distance from the earth to the sun it was about 41 astronomical units away so this actually beat the record of the previous most distant image ever taken from earth which was held by voyager 1 which is its really famous pale blue dot image where it turned around and took an image looking back towards the sun and got earth in it as well as this really pale blue dot that was taken at 40.5
astronomical units from the sun and this is 40.9 which just beats it by something like 90 million miles or something like that which is really cool obviously new horizons now beats this image every time it takes a new image because it's constantly getting further away from us however at the time this was uh the furthest image ever taken from earth which is really cute because it kind of doesn't look like much actually the wishing well cluster has had fame before again as one of these first light images that they call it when they first open up uh sort of a telescope aperture in space and it's for one of the most famous space telescopes ever it was the hubble space telescope it was the first
light image for the hubble space telescope the wishing well cluster so as much as we complain about star clusters on deep sky videos and say they're not interesting they clearly are because they're very useful for pointing spacecraft and first light images on telescopes instead of the unknown galaxy we could call it the ring bearer are you like that's the happiest i've ever been there's lots of scenarios of what could be happening one scenario that i like the sound of is called cannibalism
