Generate_series in Postgres from start and end date in a table

try this:

with dateRange as
  (
  SELECT min(start_timestamp) as first_date, max(start_timestamp) as last_date
  FROM header_table
  )
select 
    generate_series(first_date, last_date, '1 hour'::interval)::timestamp as date_hour
from dateRange

NB: You want the 2 dates in a row, not on separate rows.

see this sqlfiddle demo

You don't need a CTE for this, that would be more expensive than necessary.
And you don't need to cast to timestamp, the result already is of data type timestamp when you feed timestamp types to generate_series(). Details here:

• Generating time series between two dates in PostgreSQL

In Postgres 9.3 or later you can use a LATERAL join:

SELECT to_char(ts, 'YYYY-MM-DD HH24') AS formatted_ts
FROM  (
   SELECT min(start_timestamp) as first_date
        , max(start_timestamp) as last_date
   FROM   header_table
   ) h
  , generate_series(h.first_date, h.last_date, interval '1 hour') g(ts);

Optionally with to_char() to get the result as text in the format you mentioned.

This works in any Postgres version:

SELECT generate_series(min(start_timestamp)
                     , max(start_timestamp)
                     , interval '1 hour') AS ts
FROM   header_table;

Typically a bit faster.
Calling set-returning functions in the SELECT list is a non-standard-SQL feature and frowned upon by some. Also, there were behavioral oddities (though not for this simple case) that were eventually fixed in Postgres 10. See:

• What is the expected behaviour for multiple set-returning functions in SELECT clause?

Note a subtle difference in NULL handling:

The equivalent of

max(start_timestamp)

is obtained with

ORDER BY start_timestamp DESC NULLS LAST
LIMIT 1

Without NULLS LAST NULL values come first in descending order (if there can be NULL values in start_timestamp). You would get NULL for last_date and your query would come up empty.

Details:

• Why do NULL values come first when ordering DESC in a PostgreSQL query?

How about using aggregation functions instead?

with dates as (
      SELECT min(start_timestamp) as first_date, max(start_timestamp) as last_date
      FROM header_table
     )
select generate_series(first_date, last_date, '1 hour'::interval)::timestamp as date_hour
from dates;

Or even:

select generate_series(min(start_timestamp),
                       max(start_timestamp),
                       '1 hour'::interval
                      )::timestamp as date_hour
from header_table;