Cepheus OB1 and MisV1147 John Greaves CEPHEUS OB1 I have learnt that the open cluster NGC 7380, which lies very near on the sky to MisV1147, is also probably part of Cepheus OB 1. It certainly lies at the same distance of around 3.5 kpc (3,500 kiloparsecs). Given this, I've looked up the reddening of this cluster, that is the E(B-V) value, which is 0.56v. This means that any star seen in this cluster has a B-V value that is 0.56 times larger, that is redder, than it should be. In fact the 'v' suffix also shows that interstellar extinction is variable across this part of the sky, so 0.56 could be considered an average of a range of values, though I've no idea what the range is. Although the roughly 0.86 B-V value of MisV1147 is only reduced to 0.30 when this 0.56 is taken in to account, it is interesting. It shows that there is a great deal of extinction and reddening in that part of the sky anyway, over and above any gas and dust in the immediate vicinity of Cep OB 1. This means that only an approximately further 0.5 level of reddening would need to be assumed to get a B-V of -0.2, which would be more appropriate to an Ae/Be star. Useful considering that no nebulosity is visible near and/or around MisV1147 to, which would probably be visible if all the reddening was solely due to gas and dust at MisV1147. In summary, the natural reddening due to interstellar matter for objects at the distance of Cepheus OB 1 is around 0.56 as a consequence of all the intervening material between here and 3.5 kpc away. The reddening in the association is also variable due to extra gas and dust within the OB Association in this star forming region. MisV1147, EXTINCTION, INTERSTELLAR ABSORPTION. The following, unfortunately, is not proof of anything, because it is something of a circular argument, but it may help explain a few things. It is somthing of a circular argument because the starting assumption is that MisV1147 is a star of spectral class B5, and this assumption is based on Brian Skiff's interpretation of MisV1147's U-B and B-V via the value 'Q'. Now, E(B-V) means the excess in B-V due to so called interstellar reddening (actually deblueing, because the physical mechanism(s) involved remove(s) blue light preferentially leading to a relative excess of red light, rather than physically increasing the amount of red light). This is a measure of reddening. Thus, for MisV1147 to really be a star around B5, which would give it a true B-V of about -0.1, the measured B-V of around +0.86 would give and E(B-V) of 0.96, or approximately 1 to a rounded figure. I've now learnt that E(U-B) is related to E(B-V) by E(U-B) = 0.72 * E(B-V). Using Brian Skiff's Q info, apparently a value of 0.645 is better suited to stars of spectral class A1 to B1 than the value 0.72 is. So, if E(B-V) is about 1, then E(U-B) is about 0.65, which converts the measured U-B of -0.09 from Arne Henden to a corrected for extinction U-B of about -0.75, which is a bit blue for spectral type B5 as it should be around -0.5 to -0.6, but it looks okay. This, though, as I said before, is a little bit of a circular argument, because in many ways it is saying what Brian said but differently, that is from a different direction. Where it can come in useful though is with respect to Av, which is the interstellar absorption in visual magnitudes due to interstellar matter. Now, R is the ratio of Av to E(B-V), and the nice thing is that it seems that R is pretty much constant throughout the galaxy. That is, R = 3.2. In other words Av = 3.2 * E(B-V). It seems there are certain places where R can be as much as 5 or 6, probably in dense dust clouds, but on average and most usually it is 3.2. This is where this comes in useful. If MisV1147 is of the assumed spectral type of B5, then the measured B-V of +0.86 gives a reddening, that is E(B-V) or roughly 1, as said earlier. So, Av is simply 3.2 from Av = 3.2 * 1 These numbers are all rough, so let us just say that MisV1147 has a visual magnitude that is roughly 3 magnitudes less than it should be due to interstallar extinction. This time the argument isn't so circular, because we have visual measures of MisV1147 at maximum of around mag 13. Allowing for 3 magnitudes of extinction for MisV1147 (based on the assumption that it is a B5 star and the measured B-V points to that level of absorption) this means its true apparent magnitude would be around 10. If the object was in Cepheus OB 1 then the distance modulus (apparent magnitude minus absolute magnitude in the visual) is 12.7, or roughly 13. Apparent magnitude 10 minus distance modulus 13 gives absolute magnitude -3, which is about right for a B5 star. That's rather nice. Or alternatively, assuming MisV1147 to be a B5 star, its absolute magnitude should be around -3, and the measured B-V shows it to be reddened by around +1, so appearing fainter by +3, so its uncorrected apparent visual magnitude at maximum of around 13 eventually leads to a distance modulus of about 13 (ie [13 - 3] - -3), which just so happens to be the approximate distance modulus for Cepheus OB1. In summary, the apparent visual magnitude of MisV1147, when corrected for interstellar absorption based upon a colour excess (E(B-V)) derived from an assumed B5 spectral type, is in agreement with it being at the distance of Cepheus OB1 or A B5 star reddened by +1 at the distance of Cepheus OB1 should be about apparent visual magnitude +13, as an alternative way to look at it.