Asterisk appearing during row operation

When I'm doing row operations on matlab, and i'm trying to make a number zero, an asterisk appears. I dont know why this is.

format rat
B = [-6 8 -9 4 ;4 -3 5 7 ;-9 7 -8 -8 ;4 -3 3 3 ]

B = 
      -6              8             -9              4       
       4             -3              5              7       
      -9              7             -8             -8       
       4             -3              3              3       

B(1,:) = B(1,:)/-6

B = 
       1             -4/3            3/2           -2/3     
       4             -3              5              7       
      -9              7             -8             -8       
       4             -3              3              3       

B(2,:) = B(1,:)*-4 + B(2,:)

B = 
       1             -4/3            3/2           -2/3     
       0              7/3           -1             29/3     
      -9              7             -8             -8       
       4             -3              3              3       

B(3,:) = B(1,:)*9 + B(3,:)

B = 
       1             -4/3            3/2           -2/3     
       0              7/3           -1             29/3     
       0             -5             11/2          -14       
       4             -3              3              3       

B(4,:) = B(1,:)*-4 + B(4,:)

B = 
       1             -4/3            3/2           -2/3     
       0              7/3           -1             29/3     
       0             -5             11/2          -14       
       0              7/3           -3             17/3     

B(2,:) = B(2,:)*(3/7)

B = 
       1             -4/3            3/2           -2/3     
       0              1             -3/7           29/7     
       0             -5             11/2          -14       
       0              7/3           -3             17/3     

B(1,:) = B(2,:)*(4/3) + B(1,:)

B = 
       1              *             13/14          34/7     
       0              1             -3/7           29/7     
       0             -5             11/2          -14       
       0              7/3           -3             17/3     

Answer 1

[Matt J]

Presumably, it's just a display choice to keep things readable. The number there does exist, but they don't want to give you the mistaken impression that it is exactly zero:

B(1,:)

ans = 
       1              *             13/14          34/7     

B(1,2)

ans = 
      -1/4503599627370496

[Steven Lord]

Matt J is correct. From the documentation for the rats function (which uses the same algorithm as format rat): "Asterisks indicate elements that cannot be printed in the allotted space, but which are not negligible compared to the other elements in X."