# The Star α Camelopardalis

##### Physical Parameters of the Star
 Unit Ref Spectral Type O9 Ia 1 RA 04:54:03.011 h:m:s 2 Dec +66:20:33.58 d:m:s 2 Parallax 0.59 mas 3 $$M_v$$ -7.00 mag 4 $$v_r$$ 10 km/s 5 M★ 36.7 M☉ 4 R★ 32.5 R☉ 4 p.352 $$T_\mathrm{eff}$$ 29.5 kK 4 $$v\sin i$$ 100 km/s 4 $$v_\infty$$ 1550 km/s 4 $$\dot{M}$$ $$6\cdot 10^{-6}$$ M☉/yr 4
##### Interferometric Observation

In the 2010s the CHARA Array on Mount Wilson was used to observe O-type stars. As with Black Bodies there is a realtionship between the effective temperature $$T_{eff}$$, angular diameter $$\theta$$ and extinction corrected bolometric flux $$f_{bol}$$:

$f_{bol}=\frac{1}{4}\theta^2\sigma T_{eff}^4$

Their best fit results in $$T_{eff}=28.0\pm1.5\,\mathrm{kK}$$ and $$\theta=0.256\pm0.014\,\mathrm{mas}$$. You get $$f_{bol}$$ from model calculations [Gordon,2018].

Now let's try to calculate the stellar radius of α Cam.
With the star's distance $$d$$ and its angular diameter $$\theta$$ we can get the star's true diameter $$D$$. $\frac{D}{2d\pi} = \frac{\theta}{360^\circ}$ To get the distance we take the trigonometric parallax from Gaia. There are three data releases DR1, DR2 and DR3. The data differ significantly.

DR2 $$\mathrm{parallax}=1.3686\pm0.3297\,\mathrm{mas}$$
DR3 $$\mathrm{parallax}=0.5916\pm0.1333\,\mathrm{mas}$$.

Distances from this values are
$$d(\mathrm{DR2})=2383\pm574\,\mathrm{ly}$$
$$d(\mathrm{DR3})=5513\pm1198\,\mathrm{ly}$$.

After all the diameter of α Cam is between 20 D and 50 D.