extraction of the bright blob

I want to segment the bright blob and measure its major axis .

Answer 1

[KALYAN ACHARJYA]

grayImage=imread('image_test11.png');
bw_Image=imbinarize(grayImage,0.99); % Change threshold as per desired output
biggestBlob=bwareafilt(bw_Image,1);
imshow(biggestBlob);

# The resultant image is a mask, apply the mask to the gray image and get the result. To be more precise though, do more steps (such as morphological operations) on the gray image.