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 January 30th, 2018, 02:52 PM #1 Senior Member   Joined: Sep 2012 Posts: 201 Thanks: 1 Finding Distance of star cluster Can someone please advise me whether I have gone wrong on this question as I feel I have? Question Vega is a star of spectral type A0V at a distance of 7.76pc and has by definition $\displaystyle m_V =m B =0.0\space mag$. Observations of a star cluster show that all stars in the cluster with a spectral type A0V have an apparent magnitude of $\displaystyle m_V =15 \space mag$ and a colour index of $\displaystyle B-V=1.39 \space mag$. Assuming an interstellar reddening law of $\displaystyle A V /E(B-V)=3.2$ calculate the distance of this star cluster. Equation I used to solve m-M=5log(d/10) \space  E(B-V)=(B-V)-(B-V)_0 \space  m_v=M_v+5log(d)-5+A_v \space  My solution Step 1: To calculate the intrinsic brightness which is $\displaystyle (B-V)_0$ so I used vega to do so. I did this because according to wiki intrinsic brightness is a measure of absolute magnitude. So my reasoning is that if as the cluster and vega are in the same spectral band then they would share the same absolute magnitudes. $\displaystyle 0-M_v=5\log(7.76/10)=0.55$ Step 2: Substituting this in to my equation 2 as follows i.e. colour excess equation. $\displaystyle E(B-V)=1.39-0.55=0.84$ Step 3: Rearrange the equation $\displaystyle A V /E(B-V)=3.2$ to calculate the extinction value $\displaystyle A_v$ $\displaystyle A_v=(3.2)(0.84)=2.69$ Step 4: Using equation  the modified distance modulus and rearranging to make d the subject, I make d the following: $\displaystyle 10^{\frac{m_v -M_v+5-A_v}{5}}=d$ $\displaystyle 10^{\frac{15 -0.55+5-2.69}{5}}=2249pc$ Have I made a wrong calculation or is my assumption in the first step completely wrong, or have I complete misunderstood the question? Any advice would be much appreciated; thanks in advance. Last edited by skipjack; January 30th, 2018 at 03:25 PM. Tags cluster, distance, finding, star Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Threads Thread Thread Starter Forum Replies Last Post eila025_meys Algebra 1 May 13th, 2012 09:44 PM lalalahappy Algebra 3 May 11th, 2012 10:11 PM stevecowall Algebra 2 January 6th, 2012 03:21 AM benice Art 1 October 9th, 2011 09:31 AM ejr Economics 3 April 11th, 2011 11:02 PM

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